I'm getting 10.13m, is that correct?
Hey! By doing this question, I get a value of 3.80 meters. So something has gone wrong with the answers and your working!
An easy, intuitive way to see if you're on the right track is to visualise it. 0.9m seems way too small for the height of the cliff (this would correlate with a time of flight of about 0.32 seconds!), but 10.13m seems too big for the speed the rock (time of flight for this would be around 3.66 seconds, dropping the rock from the height of 7 meters would be around 1.2 seconds, so adding another 2.4 seconds seems a bit too much for the speed of the rock)
If you'd want to post your working, I might be able to see where you went wrong!