Well, the formula to find the area of something is base times height, so you'll have to multiply the base and height at the bottom, which is 12 times 12 which is 144, and then find the area of the missing/cut-out piece which is 8 times 5 which will be 40, and then subtract the two areas, (144 - 40), which will give you your final area of 104 square inches.
I hope I helped! =D