Halliday, Resnick & Walker — Center of Mass and Linear Momentum: Problem 20

20 At time $t = 0$, a ball is struck at ground level and sent over level ground. The momentum $p$ versus $t$ during the flight is given by Fig. 9-37 (with $p_0 = 6.0 \text{ kg}\cdot\text{m/s}$ and $p_1 = 4.0 \text{ kg}\cdot\text{m/s}$). At what initial angle is the ball launched? (Hint: Find a solution that does not require you to read the time corresponding to the low point of the plot.)