4a² - 12b = 4 (a² - 3b)
so you just pull out the common factor between the two term 4
then you divide each term by the common factor to find what remain
4a²/4 = a²
-12b/4= -3b
so now you put the four as the multiplier outside of the bracket and then put back the remainders: 4 ( a² - 3b)