I need to verify the power law in this question
There is no requirement that Vx must be positive. After you analyze a circuit you may find that any given voltage is positive or negative, and any given current is positive or negative.

What is required is that when calculating power with \$P = VI\$ you must follow the passive sign convention. If you do that, then the power calculated for any resistor must be positive...resistors always consume power. Also, power must be conserved in a dc circuit, so the calculated power values for all elements must sum to zero.