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!