Let length used for square be x, and length used for circle be L-x
Now,
Area of square A1= (x/4)^2
Area of circle A2 = (L-x)^2 / 4pi ---- Play around with 2pi.r (circumference of circle)= L-x
Therefore total A = A1 + A2 ---- Which is a quadratic in x (or L if you like)
Try to get to the maximum for A now
I'll help you more if you need it
Edit: I think it may be easier to switch around the lengths i.e. use L-x for square, and x for circle. Sorry I should have saw that coming