did you draw a diagram. always draw a diagram if possible--especially for a geometry question. if we consider just a half of the rectangle, that is, the portion on either side of the diagonal, we realize that it forms a right-angled triangle with a hypotenuse of 5 and a base of 4 (draw a diagram to see this). just find the area of that triangle and multiply it by two. if you are still having problems, get back to me.