Relate Rates - Vertical Gain

A waterskier skis over a ramp with a length of 15' and a height of 4' at a constant speed of 30 ft/second. How fast is she rising as she leaves the ramp?

Let x = the horizontal base of the ramp, y = the vertical wall, and z = the hypotenuse with theta as the angle in between x and z.

I see the following facts:

Obviously, we are trying to solve for

I tried setting the problem up like this:

I don't think this is the right answer. Does anybody see where I am going wrong?