The base of the prism is a 3:4:5 right triangle.
One of the legs is its base and the other is its height. (The longest side is the hypotenuse!)
The area of the triangular base is bh/2 = (9)(12) / 2. There are two of these.
There are also three rectangular sides on the prism.
Their dimensions are 9 by 20, 12 by 20, and 15 by 20.
You can do the arithmetic from here.
And the answer is 828cm ^2
