Question

twice the length L of a rectangle is equal to 3 times its width W. Its perimeter is 120 cm.

translate the English into math..

"twice the length L of a rectangle is equal to 3 times its width W"

2L = 3W

"Its perimeter is 120 cm."
2L + 2W = 120 but 2L=3W so
3W + 2W = 120

etc.