The only tricky thing about this problem is that you have to know
the formula for the area of a triangle. The formula is
Area of a triangle = (1/2) times (length of the base) times (the height) .
In this picture, 'x' is the base, and 'h' is the height. That's all you need
in order to find the area. You don't need to know 'y' or 'z' at all.
Area = (1/2) x (base) x (height)
= (1/2) x (13) x (6)
= (1/2) x ( 78 ) = 39 inches²