An aquarium holds 33.75 gallons of water. it has a length of 2 feet and a height of 1.5 feet. what is the volume of the aquarium?

Assuming that the aquarium is rectangular, the formula to work out the volume would be
V = L x H x D
Considering that 1 gallon = 231 inches³
Multiply both by 33.75 to get the total number of inches³ (volume)
231 x 33.75 = 7796.25 inches³

If you also want the depth, read on...

Also, there are 12 inches in one foot
Length = 2 x 12 = 24 inches
Height = 1.5 x 12 = 18 inches

Now we can fill in the information we know

7796.25 = 24 x 18 x D
To work out D, we divide the volume by the values we know
7796.25 / 18 = 433.125
433.125 / 24 = 18.046875 inches
18.046875 / 12 = 1.50390625 feet
To the nearest 2d.p. ≈ 1.5 feet