Math
posted by Riley .
I have to solve the system.
3x + y = 12
y = 9  3x
I used the substitution method to get
3x + 9  3x = 12
But now I'm stuck.

The system is indeterminate. THe two lines are parallel, and do not have a solution. Rearrange the second equation to
3x+y=9 Which is the same x,y coefficents as the first equation. 
Solving a system of equations means finding the intersection of the two lines.
Your 2 equations represent two parallel lines, they cannot intersect, therefore you cannot have a solution.