Factor the difference of the perfect square. if the binomial will not factor, write prime.
a^2-b^2

Easy peasy, this is a representation of differnce of 2 perfect squares
a^2-b^2=(a-b)(a+b)

If you have the square of two numbers that are minus each other the factor always is the minus of square root multiply by sum of the square root of them:
(a-b)(a+b) :)))
i hope this is helpful
have a nice day 

21, i'm pretty sure it's right lol