Question

Simple question that I can't figure out how to answer somehow. Factor Completely, 3ab - 39ab

Answer 1

$\bf 3ab-39ab\implies \underline{3ab}-(\underline{3}\cdot 13)\underline{ab}\implies \underline{3ab}(1-13)\implies 3ab(-12)$

one may note that both are like-terms, thus you can simply subtract one from the other, but is only needed to be factored.