Volume of a rectangular prism is length x width x height (or depth)
50 x 25.4 x 56 = 71,120 cm³ which is the total volume
We want 3/4 of that of .75 of that (same thing)
.75 x 71120 = 53,340 Jeremy should put that volume of water into his fish tank.
While I understand problems like these are designed to test your ability to properly manipulate the numbers, they often make me laugh because they are not practical.. Do you think in real life, Jeremy will use an eyedropper to fill it to exactly 3/4? What about evaporation? Will the filter pump work on the tank if the water level is that low? (My experience with fishtanks says "no")