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)