y = x + 12. The value of y's is -18.

Question

The question needs to be answered by 'solve the system of equations algebraically.'

Answer 1

Poorly presented question.  You have given one, namely y=x+12.

What, exactly, is the other equation in the system that you wish to have solved?

 

If you just want to set y = -18, then I am sure that you can solve for x, with a little bit of thought.