Please help with this question!

Two straight walls meet at an angle pi/4. A straight piece of fencing PQ of length 10 metres is used to create an enclosure APQ.

a) show that the area of APQ is given by 50(2^1/2)sin (3pi/4 - angle delta)
b) if the angle delta is varied, using differentiation or otherwise find the value of delta which maximizes the area of triangle APQ.

Thank you very much!

so what is "angle delta"?