The sides of one cube are twice as long as the sides of a second cube. what is the side length of each cube if the total volume of the cubes is 72 cm³

Side length  of a smaller cube = x
volume of a smaller cube = x³

side length  of a larger cube = 2x
volume of a larger cube = (2x)³ = 8x³

x³ + 8x³ = 72
9x³ = 72
x³ = 72/9
x³ = 8
x = ∛8
x = 2 cm  ← side length of a smaller cube

2x = 2*2 = 4 cm ← side length of a larger cube
 

The answer is 1 because you have to flip the sign when dividing by negative number and 3