#### Answer

a. 12 s
b. 50 m

#### Work Step by Step

Let the ground be level, with upward as the positive direction. The initial y is 910 m, and the final y is 150 m. The initial vertical velocity is 0. The vertical acceleration is $-9.8 \frac{m}{s^{2}}$, negative because it is directed downward. Use equation 2.11b to solve for the time of 12.45 s.
Calculate the horizontal displacement as the horizontal velocity multiplied by the time:
(4.0 m/s)(12.45 s) = 49.8 m.