3x + 7 = x - 9 {substituted 3x + 7, in for y, into bottom equation}
3x = x - 16 {subtracted 7 from both sides}
2x = -16 {subtracted x from both sides}
x = -8 {divided both sides by 2}
y = x - 9 {bottom equation}
y = -8 - 9 {substituted -8, in for x, into bottom equation}
y = -17 {combined like terms}
