area of the base (length times width) times the length of the prism (corner of the base matching to the top corner of the top base)
I hope I explained it good: Length times width of one of the bases; then save that number and multiply it to the remaining length of the prism which usually is on the side (if the prism is upright on one of its bases in problem).
Hope this helps! Have a good day/afternoon/night! :)