And what did you get for a) and b) ?
Basically I proved the equations shown in a) and b).
a) is easy, just use speed = distance/time and sub in the values.
for b) i differentiated T(x) and then subbed x into T(x) to get the equation shown in b).
If you want me to elaborate further, I guess I can take a picture of my working out to show you :/