Alright, buckle in because this will be a bit of a bore to listen to.
First, find the areas of all surfaces that you have the length and width of, which i assume you already have.
Next, use the Pythagorean theorem to find the missing measure of that triangle to find the measure of it and the back. make sure to find ALL of the sides, including the ones on the backside of the figure. Too lazy to do it myself, but at least you know how i'd do it.
