Hello,

I've attached a sketch of the window.

The area of this window consists of a rectangle and a semicircle:

The perimeter of the window is calculated by:

which is a constant of 288''. Thus:

Substitute the variable h in the first equation by this term. You'll get a function A(r). Derivate this function and solve the equation A'(r) = 0 for r.

I've got