Akite 100ft above the ground moves horizontally at a speed of 8ft/s. at what rate is the angle between the string and the horizontal decreasing when 200ft of string has been let out?

So hmm notice the picture

at the moment r = 200, or the string is 200ft to the kite up above, at that moment, those are the values for "x" and θ

however, "r" has a rate, so.. we wont' be using that one to get the angle, we'll use the "constant y" or 100

thus   

so hmm, pretty sure you can take from there, keeping in mind that dx/dt is 8