perimeter is all sides added up together. So as you don't know a side's length you can put it as x and make an equation.
x + x + x + 6 + x + 6 = 180
4x +12 = 180
4x = 180 - 12
4x = 168
x = 168/4
x = 42
as length is 6 feet longer than width length is equal to 42 + 6 = 48 feet
