Think of the shape as being three rectangles stacked on top of each other. Lets call the top shape r1, the middle shape r2, and the bottom shape r3.
We know that all of them together have a height of 15 ft, the height of r1 is 4 ft and the height of r2 is 5 ft. So the height of r3 must be 15-4-5=6ft.
We know that all of them together have a width of 18 ft, width of r1 is 1 ft, and the width of r3 is 18 ft. The width of r2 must equal 5 ft plus the width of r1, 5+1=6.
Finally we use the equation for the area of a rectangle, Area=base(width)*height. Find the area of each individual rectangle and add the three areas together to get the area of the whole shape.
