Can you please help me with this math problem

This question is nasty. It sounds like you should be finding area of the figure you see. I'll do that first, although I don't think it's the right answer. (But it might be).

Area Trapezoid = (b1 + b2) * h / 2
b1 = 8
b2 = 12
h = 5
Area Trapezoid = (8 + 12 ) * 5 / 2
Area Trapezoid = 20 * 5/2
Area = 50 square feet.

The problem is that no lumber yard will cut that for you without charging you. The practical answer is 12 * 5  = 60 square feet is what you will have to buy. Then you cut out the triangles for yourself.