From Förberedande kurs i matematik 1

If we look at the expression, we see that it can be written as \displaystyle x^2-6^2 and can therefore be factorized using the conjugate rule

\displaystyle x^2-36=x^2-6^2=(x+6)(x-6)\,\textrm{.}

Because the factors \displaystyle x+6 and \displaystyle x-6 are linear expressions, they cannot be factorized any further (as polynomial factors).