(b) asks you to "use function notation" to express the question "When is the car going 70 mph". On the left side of the equal sign you have "v(t)". Were you given that or did you choose it? Often "v(t)" is used to indicate the speed or velocity at time t but there is nothing in the problem that defines v(t). The only thing defined here is s(t). The speed, at time t, is the derivative of that, s'(t). I would answer s'(t)= 70.