Solving equations {3x-4y = - 12 {y-3x = 9} by substitution method {3x-4y=-12 {y-3x=9

Solving equations {3x-4y = - 12 {y-3x = 9} by substitution method {3x-4y=-12 {y-3x=9

y=9+3x ①
3x-4y=-12 ②
① Substituting in 2:
3x-4(9+3x)=-12
-9x=24
x=-8/3
So y = 9-8 = 1