7ab-3a^2b^2+7+8ab^2+3a^2b^2-3-7ab
=(7ab-7ab)+(3a^2b^2-3a^2b^2)+8ab^2+(7-3)
=8ab^2+4-1/2a^2ab+1/2cba^2-1/2a^2ab+1/2cba^2=-1/2a^3b+1/2cba^2=1/2a^2b(c-a)