A rain gutter is to be folded into an isosceles trapezoid from a piece of tin 20 inches long. The base angles are to measure 150 degrees. Find the maximum carrying capacity.

I have my picture drawn, everything labeled, and I found the area of both triangles and rectangle (of the trapezoid). this is the function I got:

f(x) = 10x - x^2 + (x^2?3)/4. where do I go from there???