Factorise completely 9a^2-1

(3a-1) (3a+1)

Step-by-step explanation:

This is the difference of two squares so we can factorise using the rules

x^2-y^2 = (x-y) (x+y)

In this case x = 3a and y = 1 since (3a) ^2 = 9a^2 and 1^2 = 1