check the picture below.
so the area of the rectangle is x*y.
the area of the triangle is (1/2)(w)(z).
if we subtract the area of the triangle, from the containing rectangle, we're in effect making a whole in the rectangle, as you see there in lower-right corner.
[tex] \bf \stackrel{rectangle}{x\cdot y}~~-~~\stackrel{triangle}{\cfrac{1}{2}wz} [/tex]
