Check the picture below.
so the shape is really just a rectangle with a triangular cutout.
well, the rectangle is just a 8x13 rectangle, and the triangle has a base of 5 and a height of 6, now, if we get the area of the rectangle and then subtract the area of that triangle, what's leftover, is the shaded area, the part that wasn't subtracted.
[tex]\bf \stackrel{\textit{\Large Areas}}{\stackrel{rectangle}{(8\cdot 13)}~~-~~\stackrel{triangle}{\cfrac{1}{2}(5)(6)}}\implies 104-15\implies 89[/tex]
