Question

I don't know how to do this Y=x-7 3x-3y=21

Answer 1

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!