Calulating the total thermal power in three phase system
The U' voltages supply the electric radiators. They can be used to calculate the power of the electric radiators, but the current can be calculated and the load power calculated using the current and load resistance.
The voltage at the source end of the transmission line must be used to calculate the current since the impedance and losses in the transmission line are part of the problem.
The currents are calculated using the line to neutral voltages from the source end of the transmission line and the transmission line plus load resistances. Since the system is completely balanced, the neutral voltage and current are zero.


I believe that sufficient information is given to work the problem more than one way, but I have not looked at it thoroughly since the question didn't really ask about that.