Both of these use the intermediate value theorem (IVT).
For 7), note that g(0) = f(0) – 0 = f(0), which is in [0, 1], since the range of f is in [0, 1]. If f(0) = 0, we have a suitable c of c = 0.
So suppose g(0) = f(0) is strictly positive instead (this is the only other possibility...