Question

how to solve for x: 2x+y=3 and x+y=9

Answer 1

The answer for the first one would be: y=3-2x
The answer for the second one would be: y=9-x

Answer 2

X+Y=9; X=9-Y; 2X+Y=3; substitute above equation for X; 2(9-Y)+Y=3; 18-2Y+Y=3; 18-Y=3; 18=3+Y; 15=Y; X+Y=9; X+15=9; X= -6; 2X+Y=3; 2(-6)+15=3; -12 + 15 = 3; 3 = 3;