I don't know how to do this

Y=x-7

3x-3y=21

You're going to use the substitution method for this.

First, we know that Y = x-7, so we can plug x-7 into everywhere we see Y!

3x - 3 (x - 7) = 21

then we simplify

3x - 3x + 21 = 21

^ ^

These cancel out!

So we're left with ...

21 = 21

That means that the solution to this equation is all real numbers!