Your suitcase is shaped like a rectangular solid. To determine its volume, multiply length by width by depth. The unit of the answer is cubic centimeters. One liter is 1000 cubic centimeters.
V = 50 * 37 * 20 = 37,000 cm^3
This is exactly 37 liters.
I know you asked about math, but "it's" is short for "it is". When you want to show the possessive form of the pronoun "it", there is no apostrophe. The word "its" means belonging to it.
"Its measurements are..."
50 cm × 37 cm × 20 cm = 37,000 cm³