Question 107332
1.{{{x+y=12}}}
2.{{{y=3x}}}
You have two equations with two unknowns, a linear system of equations. 
You should be able to solve for x and y.
Using equation 2 and substitute into equation 1.
1.{{{x+y=12}}}
{{{x+(3x)=12}}}
{{{4x=12}}}
{{{x=3}}}
Now substitute into 2.
2.{{{y=3x}}}
{{{y=3(3)}}}
{{{y=9}}}
Check your answers with your original equations.
1.{{{x+y=12}}}
{{{3+9=12}}}
{{{12=12}}}
True statement. Good answer. 
2.{{{y=3x}}}
{{{9=3(3)}}}
{{{9=9}}}
True statement. Good answer. 
x=3 and y=9.