Answer
28
Explanation:
you have an area of 20, which is a clue to find two numbers that could multiply to get 20. Then, to be sure it is the right pair of numbers, you will add them twice to see if you get 18 as the perimeter. I found the two numbers 5, and 4. I added 5 to 4, and added 9+9+5+5, and got 28.