Answer
a) $7.3\text{ ft/s}^2$
b) $t=0$
Work Step by Step
a) We consider the tangent line at $60\text{ mi/h}$. Two points on it may be $(0,20)$ and $(16,100)$. We determine the acceleration:
$$a=\frac{v_2-v_1}{t_2-t_1}\cdot\frac{88}{60}=\frac{100-20}{16-0}\cdot\frac{88}{60}\approx 7.3\text{ ft/s}^2.$$
b) We notice that the acceleration is maximum at $t=0$.
