Power dissipated between two points
The voltage sources must dissipate the difference between the total power (210 watts) and the power dissipated by the three resistors (180 watts). The power difference is therefore 30 watts and, given that 3 amps is flowing, that must mean that the net voltage across the two batteries (if they were combined as one) is 30 watts ÷ 3 amps = 10 volts.
To have a net voltage of 10 volts requires \$V_B\$ to be 20 volts i.e. there is 20 volts in opposition to the 30 volt battery. Or, you could make an argument to say that \$V_B\$ is 40 volts because it still produces a net voltage of 10 volts when considered to be in series opposition with the 30 volts battery. What you have to decide is what is meant by power dissipation; is it net power supplied by the batteries or net charge power received by the batteries.
I can't answer that based on the details in your question.