When you subtract one square number from another = 5

Perfect squares:

1^2 = 1

2^2 = 4

3^2 = 9

4^2 = 16

5^2 = 25

6^2 = 36

...

Which of these perfect squares are 5 units apart?

Well, simply do the subtraction to find out.

36 - 25 = 11

16 - 9 = 7

9 - 4 = 5

Ah we found the squares! 9 and 4.