Factorising the difference of two squares
This comes up in every exam, at every level. You just have to know it, and recognise it when you see it.a2-b2=(a+b)(a-b)
It is easy to verify, if you multiply the brackets to go backwards.
It is quite easy to remember.
It can even help you with arithmetic: e.g. 11 x 13 = (12-1)(12+1) = 122-12=144-1=143