Here we have to multiply 3b +3 and b+7 and for that we use distributive property which is (a+b)(c+d)= ac + ad +bc + bd . And on using that rule , we will get
(3b+3)(b+7) =3b(b+7) +3(b+7)
= [tex] 3b^2 +21b +3b+21=3b^2 +24b+21 [/tex]
And that's the required result .
