Getting much deeper than OP probably needs for VCE but i cant resist. "I think", in my view, is probably better actually replaced by "This thought exists" or "A thought exists", in my opinion anyway. (Shameless plug, there is the philosophy thread in my signature down there, check it out, it's currently hibernating but i think ill kickstart it soon).
It also raises many other questions of what exactly we call ourselves or what makes us ourselves. If you ask a buddhist, they would say we don't really exist as a person. They don't deny we have a continuous address or phone number. They don't deny if i stand in-front of you, i will be present. However, the idea we're one constant person, according to them, simply isn't true. We're always in flux and change in every moment to the next. Therefore, in every moment to the next, we are different people. Past kingpomba was a different guy in some sense of the word "different", he wasn't me. Future kingpomba isn't either, so, that's also worth thinking about.
Personally, I always thought that the reverse of Descartes' formulation made more sense: "I am, therefore, I think".
(Before i say anymore, i think descartes little line has been massively blown out of proportion and most people just kind of accept we exist ipso facto and don't really worry or attempt to prove it).
It works in some ways but it other ways it kind of doesn't imo. It implies that if you exist, you must think or that thinking is a necessarily outcome of existing. There are people who are in coma's who exist (i suppose you could argue they don't but people do sometimes wake up) but do not think. My lamp exists for instance but does not think. So, what i think you're really saying here is that humans exist and this follows they think, this doesn't really give to us any new information about whether we exist though, all it really shows us is that humans apparently think.
If you lay it out in the way often used in teaching (more expanded than it should be):
P1: I exist
P2: If i exist i must think
C: I think
All that formulation really concludes is that we think. It takes for granted or just blindly states that we exist as one of the premises, when of course, OP is trying to determine whether we exist and it isn't taken for granted. If you reformulate Descartes version in a similar way:
P1: I think (or, this thought exists)
P2: The existence of a thought implies the existence of a thinker
C: I exist
Hopefully laying it out this way kind of illustrates the crucial difference. One assumes we exist and concludes we think (that is its discovery or outcome), the other assumes thoughts exist and concludes we exist. In the Descarte version, P1 (I think/This Thought exists) seems kind of axomatic, it must just simply be true, it's very hard to reject this and conversely, very easy to accept this as an uncontroversial premise. In the alternative version, P1 (I exist), is very far from axiomatic or granted, so, it jerpodises the conclusion. Say i make an argument proving Jesus is divine with the Christian God existing as one of my premises, most people would simply just reject that premise and consequentially, the entire argument is invalid in there eyes. There's a danger in using controversial items as premises (not that it's necessarily wrong or bad of course, happens all the time in philosophy of religion and often ethics).
In the case of the stone, i imagine descarte would likely reply we can't actually be sure the stone exists. His phrase was authored in the context of radical doubt, not so much attempting to prove that he exists or anything exists but rather to show we can be sure there is at least one thing that doesn't not exist (i know this wording may be confusing). He pointed out we could doubt leaves, socks or stones exist, we could doubt our friends or even other people exist. There is almost nothing we can be sure of but it seems we can be sure of the fact that our minds exist. So, it was not so much an exercise in proving we exist as in proving that we can be sure something doesn't not exist.