(6c^-2cd)+(10c^+18cd)=
(14mn-15m)+(-15mn-14m)=
(3a^-7b^)+(6b^-2a^)=
(6c^-2cd)+(10c^+18cd)=16c^2-16cd
(14mn-15m)+(-15mn-14m)=-mn-29m
(3a^-7b^)+(6b^-2a^)=a^2-b^2