Answer
a. Start with the equation at the top of page 42.
$$L = v_{average}\times time$$
However, from the discussion on page 48, we know that the average speed under constant acceleration is the average of the initial and final velocities.
$$v_{average}=\frac{0+v}{2}=\frac{v}{2}$$
Combine the above equations.
$$time = \frac{L}{v_{average}}=\frac{L}{v/2}=\frac{2L}{v}$$
b. Plug in numbers.
$$time =\frac{2L}{v}=\frac{2\times1.4m}{15.0m/s}=0.19s$$
