Let's turn this into an equation. Since we use 2L+2W=P to find the perimeter, we need to use that equation but fill in the information we know.
2(18-2x)+2(x+9)=38; let's distribute the parenthesis. 36-4x+2x+18=38; let's add like terms. 54-2x=38; let's subtract 54 from both sides. -2x=-16; divide both sides by -2. x=8 Let's check to make sure 8 is the correct answer. We need plug it in to both sides.
x+9=8+9=17; that's the length of one side, let's find the other. 18-2(x)=18-2(8)=18-16=2; that's the length of the other side.
17+17+2+2=34+4=38
So, x=8 ft.
