Find the surface area of the prism? HELP I need the steps if possible

To find the surface area of a figure, find the area of all the shapes that make up the figure to get the area multiply Base by the height. First let's find the area of the rectangle  and the front that makes the ramp. To do this multiply 20 by 17 to get 340. Then let's get the area of the rectangle on the bottom by multiplying 15 by 20  to get 300. Now lets find the area of the rectangle in the back we will do this by multiplying 8 by 20 get 160. To get the area of a triangle multiply the base by the height and then divided by 2. so for one of the triangles let's multiply A perfect in which gives us 120 and divide that by 2 to get 60. because the other triangle has the same dimensions, the area of the other triangle will also be 60. now let's out of all the areas that we have from the shapes  which will be  340 + 300 + 160 + 60 + 60 which will give us 920 so the area of the figure is 920ft^2

(can i get brainelist pls)