First I made equations for the perimeter (p) in terms of the width (w) and same for the length (l).
Then, since all the sides added together equaled the perimeter, I made the perimeter equation equal to all the sides added together, all in terms of width (no other variables than w).
Solving that algebraically, you can find w. And then plug that back into the equation for length to find length.
w=8, l=11