OK, you have in part. But you have missed some significant questions. (1) Is f(x)=x+x+x differentiable, or not? You have answered this: Yes. Now please answer the next two questions. (2) If it is, what is its value? (3) Does the value differ when x=3 from when x!=3? Also please answer questions 2 and 3 for f(x)=x and x=1, and for f(x)=x+x and x=2.