Factor the algebraic expression 27a+21

We know that we can do

ab+ac=a (b+c)

so find common factor

factor them

27a=a times 3 times 3 times 3

21=3 times 7

common factor=3

3=a

27a/3=9a

21/3=7

so now we have

(3) 9a + (3) 7=3 (9a+7)