let x = -2
f(-2) = -5(-2) +3
f(-2) = 10+3
flip each 45-45-90 triangle inwards (we can do this because it is a square and squares have all four sides equal). you should get a plain old shaded square, which means that the shaded square is half of the area of the square with side length 16 cm. so, we have (16 cm)^2 or 256 cm^2, which, when halved, is 128 cm^2 or b.
note: is it a coincidence that it is half? does this always happen? ; )