Balancing Power in a Simple Circuit When not all Wattages are known
You know P1 and you know the current in P1 so you can calculate the bus voltage trivially. 

Once you have the voltage then power levels for 2,5 are trivially derived because you know the currents. 

Do the (signed appropriately) addition and you get the power in 3 + 6, from which you can get Ix (and if desired) the power P3 alone.