Take into account that the given figure can be takes as a rectangle and a right triangle together. Then, the area of the figure is the area of the rectangle plus the area of the right triangle.
For the rectangle you have:
A = (20 ft)(18 ft) = 360 ft²
For the right triangle:
A' = (20 ft)(24 ft - 18 ft)/2 = 60 ft²
A figure of the image is shown below:
Hence, the are of the figure is:
AT = A + A' = 360 ft² + 60 ft² = 420 ft²
