ok thank youu
For this question how do you know its SN1 or SN2 for b
Woah - who's asking you this? This here be some uni-level chemistry. And not just any level, this isn't taught until 2nd year organic chemistry at Monash. Some places likely teach it in first year, but definitely not VCE level (unless I'm reading the study design wrong). I wouldn't worry about being able to answer this (goes double for anybody reading this and scared by the terminology because they'd never seen it before)
The quick way to identify which is which is to know that SN1 reactions can only occur if there is a stable intermediate for the reaction to progress through. If the intermediate isn't going to be stable, then the reaction will be SN2. Remember, the first step in SN1 is the leaving group leaving - for both molecules, this would be the bromide. If the bromide leaves the first molecule, you're left with a primary carbocation - not particularly stable, and so an SN1 mechanism is very unlikely, and it's more likely to undergo SN2 - like you've suggested. For the second reaction, however, the leaving of the bromide creates a tertiary carbocation - much more stable, so an SN1 mechanism is much more likely.
Another thing you can look at is sterics - or how bulky the neighbouring groups are. This rule is a little more complicated than the carbocation rule (note: looking at the stability of the intermediate isn't always going to involve carbocations, it's just that in this instance, that is what we've ended up looking at) I've used above. If you look at the first molecule, look at the carbon that the SN2 reaction would happen to. It's the one connected to the bromide. For an SN2 reaction to happen, the nucleophile needs to be able to attack that carbon while the leaving group (the bromide) is still attached. Because the molecule is nice and linear, there should be enough physical space available for the nucleophile to attack (try imagining the molecules in 3D and how you'd expect them to twist around and behave). However, for the second molecule, do you see that "V"-shaped group that's attached to the same carbon as the leaving group? That thing will likely get in the way of an SN2 reaction - whereas an SN1 reaction will have more room, because when the nucleophile comes in, the leaving group is already gone. As a result, we would say that the SN2 pathway is sterically hindered, and so you'd expect to see an SN1 mechanism as a result.
An important thing to remember - these reactions aren't necessarily "this mechanism, not that mechanism" - both can and will happen. If you've drawn a mechanism that makes chemical sense, then the reaction can proceed that way. The question is which mechanism is more likely. For example, in the case of the first molecule, the SN2 mechanism is much more likely, but the SN1 mechanism can still occur. It's highly likely if you were to watch the reaction play out, 99% of the molecules will undergo an SN2 mechanism, but 1% might still undergo an SN1 mechanism. When it comes to mechanisms, there are no hard-and-fast rules, because there's not even necessarily a correct answer at the end - just what we think is most likely. As a result, you might make a mechanism that is consistent with all the rules you know, but still get the question "wrong" because one of the steps is more likely to go a different way than you thought it would. Try not to be discouraged - it's just a matter of practice makes perfect, and you'll eventually learn to realise all the different pathways a molecule can take.