3x-y=7
9x-3y=21
9x-3y=21
Put the equation into slope intercept form so that you can later substitute it into 3x-y=7.
9x-3y=21
-9x. -9x
-3y=-9x+21
Divide each number by -3.
You get y=3x-7
You will substitute y for 3x-7 in 3x-y=7
Solve:
3x-(3x-7)=7
3x-3x+7=7
0+7=7
7=7
This means that both equations are the same.