The question as presented is rather obscure. The AREA of the wall?
Presumably, the angles at the corners are 90 degrees. And presumably C is where [MATH]\theta[/MATH] is indicated, but it would be nice to be told that.
If all those guesses are correct, I am missing what is difficult about the problem. The radius of the circle is
[MATH]\sqrt{15^2 + 5^2} = \sqrt{225 + 25} = \sqrt{25 * 10} = 5\sqrt{10}.[/MATH]
[MATH]\theta = \pi - 2 \alpha.[/MATH]
[MATH]\alpha = arctan \left ( \dfrac{5}{15} \right ) [/MATH],
and so on.