Find the voltage Vab and current of the circuit I
Use your eyes and very simply, write down the numbers for voltages and the currents defined by ohm's law: -

I think your big mistake is not recognizing that ground (shown in green) connects to the 6 volt source and the 5 ohm resistor. Do not launch into anything mathematical until you understand the diagram and it is simplified.
So, quite literally the voltage at node a becomes -6 volts and, the voltage at node b becomes -20 volts (just by visual inspection). These voltages define the current of 7 amps flowing through the 2 Ω resistor.
Node b being at -20 volts also defines the current through the 5 Ω resistor being 4 amps. The 7 amps and the 4 amps tell you that 11 amps flows through the 20 volt battery.
-6 volts across the 3 Ω resistor means there is 2 amps flowing in it.
And finally, you should be able to see why the 5 amps flows in the 6 volt battery. This bit is down to you.
Simple visual inspection of the circuit and ohms law.