7x+4-1=0. 4x+2x+1=80.
7x+3=0 7x=-3 x=-3/7 6x+1=80 6x=80-1 6x=79 x=79/6=13 1/6
7x=1-4 7x=-3 x=-3/7 _____________ 6x=80-1 6x=79 x=79/6