Question

A particleis moving in a staight line so that its position is given by the relation x = (2.10 m/s^2)t^2 + 2.80m. Calculate its average acceleration during the time interval from t1 = 3.00s to t2 = 5.00 s and its stantaneous acceleration as a function of time.

Answer 1

I already have the answer (this is from an example in my book).I'm just confused on how they got the answer, that is for the velocity at any time:

(2.10 m/s^2) t^2 + 2.80m = 4.20 m/s^2 t^2