For the triangle, the shaded part, both the height and base are 12, so the area of it is:
1/2 (12 x 12) = 72
And the area of the whole square is 144
But what we’re looking for is the area of the unshaded region. So to find it, we’ll need to subtract the shaded from the total. So:
144 - 72 = 72
Thus, the area of the unshaded region is 72
