Question

Rectangular fish aquarium with height 15 inches length 20 inches and width 20 inches

Answer 1

If you're trying to find the volume then you'd multiply your Length× Width× Height. so V= L×W×H. By plugging in your numbers it would be.
V= 20 × 20 × 15. All that multiplied would be equal to.. 6,000 cubic in. That would be your formula.

If you're looking for area then it would just be..
$a = l \times w$
which is 20 × 20 = 400 square in.