I need help with this system of equation question

I was able to get it right but don’t understand. I either have to use substitution or elimination. I used substitution and can only get the begening (3, 11) by subsisting one into the other and putting the x back in to get y.

How do you get the -3? I’ve looked it up and people get the -3 from x^2=9 but I only get 3.

35 points.