You are helping a partner in your group with their problem. What mistake did they make solving the system?
3x+y-9
-2x+y=4

Students work when solving the second equation for y:
Y=2x+4
-2x+(2x+4)=4
-2x+2x+4=4
4=4

The students response is there are indefinitely many solutions and this is not correct. Explain the mistake.