Question

3x-y=7
9x-3y=21

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

Answer 1

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.