The lenght of a rectangle is twice its width. The area of the rectangle is 18 square centimeters. Find the perimeter of the rectangle?

Let x = width
2x = length as given in the description

Area of a rectangle is length * width so we do:

2x(x) = 18
2x² = 18

Divide by 2 to isolate our variable:

x² = 9
√x² = √9
x = 3

Length = 6, width = 3

Perimeter = (Length * 2) + (Width * 2)

We plug in our new information to find the perimeter.

P = (6*2) + (3*2) 
= 12 + 6
= 18

The perimeter of the rectangle is 18 centimeters. Ironically, matching its area.