A student is watching a remote-controlled helicopter land atop a tree stump that is 9 m away. Using a motion sensor, the student calculates the helicopter is falling at a rate of 18 m/s when it is 5.2 m above the stump. At this moment, how fast is the angle of elevation changing relative to the student’s line of sight?

–1.50°/s

–1.73°/s

–1.98°/s

–2.00°/s
I know the answer is -1.50 degrees/s, but could someone help me understand why? I tried myself but I got 0.21 degrees/s.