Determine the value of Vx if the power dissipated by resistor Rx is 450W and Rx=50 Ohms
Try calculating the current flowing through resistor Rx (hint - P = I^2 R - and then using this current to find the 1) voltage dropped on the 10ohm  resistor below it, and also the voltage dropped on Rx.   Then use kirchoffs voltage law to find the missing Vx.