A plane flies horizontally at an altitude of 5 km and passes directly over a tracking telescope on the ground. When the angle of elevation is is pi/3, this angle is decreasing at a rate of pi/6 rad/min. How fast is the plane travelling at that time?

http://i.imgur.com/mbKzq.png

Now I need to find 'y' which is impossible because I only have one given side and one angle. How do I solve the rest?