Which system of equations has exactly one solution?
2 x minus 4 y = 8. x + y = 7.
3 x + y = negative 1. 6 x + 2 y = negative 2.
3 x + 3 y = 3. Negative 6 x minus 6 y = 3.
3 x minus 2 y = 4. Negative 3 x + 2 y = 4.