-7а-9b+a+11b=-6a-9b+11b=-6a+2b
a-a+8+(12+a) =8+12+a=20+a
-3(c-5) +6(c+3)=-3(c-5-2(c+3))=-3(c-5-2c+6) =-3(-c-11)
-7а-9b+a+11b=-6a-9b+11b=-6a+2b
a-a+8+(12+a) =8+12+a=20+a
-3(c-5) +6(c+3)=-3(c-5-2(c+3))=-3(c-5-2c+6) =-3(-c-11)