[tex]y=36-9x[/tex]
[tex]3*x+\frac{y}{3}=12[/tex]
substitution of y in 2nd by right side of 1st equation:
[tex]3*x+\frac{36-9x}{3}=12[/tex]
[tex]3*x+12-3*x=12[/tex]
[tex]12=12[/tex]
This is identical equation which means there is infinite number of solutions, make sure the symbols in equations are right.