To find the volume of any prism you first need to find the area of the cross-section and then multiply that by the depth. To find the area of a triangle, as that would be the cross-section in a triangular prism, you do (base × height)/2. Then you multiply that value by the depth and that's the volume. In case you're unsure, the base of the triangle would be the base of the visible triangle at the front, as would the height, and the depth would be the remaining value.
I hope this helps!