Answer
The length of the dog pen is $12$ ft and the width of the dog pen is $6$ ft.
Work Step by Step
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