Answer
$(3x+6)(x+6)-3x^2$ = 396
$24x+36 = 396$
width is 15 feet, length is 45 feet
Work Step by Step
length = 3x width = x
3 foot barrier all around $(3x+3+3)(x+3+3)$
$(3x+3+3)(x+3+3)$
$(3x+6)(x+6)$ is area of entire space
$3x*x = 3x^2$ is area of pavilion
$(3x+6)(x+6)-3x^2$
$3x^2+18x+6x+36-3x^2$
$24x+36$
$24x+36 = 396$
$24x+36-36 = 396-36$
$24x = 360$
$24x/24 = 360/24$
$x = 15$