Question: how to find the perimeter when you know the area of a rectangle?
Answer: Remember the formula for perimeter and area of a rectangle. The area of a rectangle is a = length * width, while the perimeter is p = (2 * length) + (2 * width)
Substitute the known values into the area formula. 36 = 4 * w. ... 
Substitute values for length and width into the perimeter formula.