c+2d=6
3c6d=18
to eliminate one variable we can multiply times 3 the first equation and add to the second equation
3c+6d = 18
3c  3d=18
The two equations are the same thing and the system is undetermined.
The determinant of the system is Zero so the system is undetermined. This is a so called inconsistent system.
7/6/2014

Francisco E.