How do you factorise and know if it is a difference of two squares ?

1. First you need to learn how to recognise the form:

Difference of two square numbers.  a² - 36

(a² is a square number and 36 is also a square number and you are getting the difference between them)

To factorise, you would need to be put in to a form of

(a+b) (a-b)

2. Two numbers of a and b in the bracket remains the same, but the one bracket is the sum (+) and the other is the difference (-).

For example: a² - 36

Two numbers would be a and 6.

(a+6) (a-6)  And one would be (+) and the other would be (-)