First let’s define our variables
P= perimeter
L = length
W= width
And then with the formula for the perimeter of our rectangle
P = 2L+ 2W
We then can substitute what we do know that; P=18 and the W=4
18 = 2L+ 2(4)
Then solve by isolating the variable
18 = 2L+8
18-8 = 2L +8-8
10 = 2L
10/2 = 2L/2
L = 5
So our length is equal to 5 feet.