The order of the factors does not matter.
So (a-b)(a+b) is the same as (a+b)(a-b)
This is known as the difference of squares factoring rule.
You can FOIL out (a-b)(a+b) and you'd get a^2+ab-ab-b^2. Note how the ab terms cancel leaving us with a^2-b^2. This confirms we have the correct factorization.
