This is a question where I am troubled to calculate the thevenian voltage.The original question is to calculate the current through 10 ohms resistor.

 

Try using superposition. Take each voltage source individually assuming the other is a short circuit. Then add the two values to give the result. Be careful with the source polarities.

 

First remove the 10 ohm resistor and calculate the voltage across AB. This the thevenin voltage.

Then remove the 10 ohm resistor and short all voltage sources and calculate the resistance a multimeter would measure if it was placed at points AB. This is the thevenin resistance.