Chapter 9 - Basic Algebra - 9.8 Using Equations to Solve Application Problems - 9.8 Exercises - Page 691: 51

The length of the dog pen is $12$ ft and the width of the dog pen is $6$ ft.

Let $x =$ the width of the dog pen $2(2x) + 2(x) = 36$ $4x + 2x = 36$ $6x = 36$ $x = \frac{36}{6}$ $x = 6$ ft Let $y = $ the length of the dog pen $y = 2x$ $y = 2(6)$ $y = 12$ ft