Answer :

[tex]a.\\18=9\cdot2;\ 63=9\cdot7\\\\18+63=9\cdot2+9\cdot7=\boxed{9\cdot(2+7)}\\-----------------------------\\\\b.\\84=7\cdot12;\ 35=7\cdot5\\\\84+35=7\cdot12+7\cdot5=\boxed{7\cdot(12+5)}[/tex]