Hey I've been trying this problem forever and it's not working if I could have some help that would be Awsome

A plane flies horizontally at an altitude of 4 km and passes directly over a tracking telescope on the ground. When the angle of elevation is , this angle is decreasing at a rate of _ Fad/min. How fast is the plane traveling (in km/min) at that time? (Round your answer to two decimal places. ) 4 km/min