We know the perimeter of a square is 4 times the length of one of it's sides. So
36 = 4 * (9x)
36 = 36x
First write down what you know
1) a perimeter is the sum of the length of all sides
2) a square has 4 sides
3) All sides in a square are equal so.....
we can write that 36= 4 * (9x)