3x-y=7

9x-3y=21

how do I do it by the substitution method i think so.?

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.