Here is the question I am having trouble with:

The sun is passing over a 100 m tall building. The angle θ made by the sun with the ground is increasing at a rate ofpi/20rads/min. At what rate is the length of the shadow of the building changing when the shadow is 60 m long? Give your answer in exact values.

So far I have got:

100cosfata=x

dx/dt=-100(cosfata)(dfata/dx)

Dont no where to get cosfata from.