To find the perimeter rectangle you would add the measures of the outside sides of the rectangle.
For instance, if the lenghths of the sides of the rectangle are 4,4,5, and 5, then you would add like the following:
4 + 4 + 5 + 5 =
You would find then that the perimeter of the rectangle would eaqual 18 units.*
*Units stands for whatever label your rectangle is measured with.