#### Answer

The launch angle of the golf ball was $45^{\circ}$.

#### Work Step by Step

For a ball launched moving up at an angle $\theta$ to the horizontal, it also lands moving down at an angle $\theta$ to the horizontal (in Figure 3.11).
The rotation of the velocity vector is thus given by the sum of the two angles above - $2\theta=90^{\circ}$.
Therefore, the launch angle, $\theta$ of the golf ball was $45^{\circ}$.