To find volume, you need to multiply the height width and length together. So when you have one missing measurement, to find it, you need to do the same steps except for put a variable for the missing measurement. And continue to do the problem as you would. Here is how I solved it.
V=12x18xH
3024=216xH (216 is 12x18)
Then to find the missing side, do 3024/216 which equals 14.
