Potential to earth with 3 power sources
For C you have 14V and 6V in series making 20V. So the current through the resistors is 1A. making a voltage drop of 10 V over each resistor. 
So the voltage at c would be 14 -10 = 4V. Or along the other path 10 -6 = 4V also. Look at the polarity of the sources so you can correct the error. Thereafter it is easy

The 8 V source is not inside any circuit so no current can flow.