Here's my question...

A Norman window, one that is formed by placing a semicircle on top of a rectangle, still remains a popular architectural feature. If the perimeter of the window is 300cm, find the radius of the semicircle that will maximize the window's area (let in the most light).

I came up with this equation... 300 = 2x + y + ((pi*y)/2)

x = rectangle height

y = rectangle width

These questions really bug me and I'm really stuck! Help please!