You just have to imagine it as a rectangle, with a square.
The rectangle inside is 18" by 8".
The square, made of the outside two triangle pieces, is 8" by 4". (Yes, I know it isn't really a square.)
Area = length x width.
A1 = 18 x 8
= 144.
A2 = 8 x 4
= 32.
A1 + A2 = 176 square inches.
