Solve for x.
The power theorems.

I count three rectangles and 2 triangles.

Top rectangle 15×12

Bottom rectangle 15×13

Back rectangle 15×5

The triangles are right triangles, sides 5,12,13.  So the legs are base and height

Two Triangles     2×(1/2)×5×12

Adding it all up

A = 15×12 + 15×13 + 15×5 + 2×(1/2)×5×12

A = 15(12 + 13 + 5) + 5×12

A = 15(30) + 60 = 17(30)

A = 510

510 square feet