To solve this problem, you have to use the passive sign convention in order

to be able to assign the polarities across a voltage drop of the elements.

So in this case, we have a voltage drop across the resistor that is

unknown as far as the polarity goes.

So we first assign a direction for the current.

And then, knowing the direction for the current that we've assigned,

we can then assign the polarity across the resistor.

In the passive sign convention, the current is assumed to enter the positive

side of the voltage polarity across the element.

So if we assign the current to go clockwise around this single loop circuit,

then the voltage drop across the resistor,

P sub R is going to be, as shown in the circuit now, and

ultimately we're going to decide what the resistance is and

what the voltage source of this V sub S is?

So we know that using the equation for

power, that power = I squared R.

And we're given the power in this problem.

The power in the resistor is equal to 80 milliwatts.

We're also given the current, the current is equal to 4 milliamps.

And the problem is on the left-hand side as a current source.

So we have everything we need in order to solve for the resistance.

So the resistance is going to be equal to P/I squared,

and P=80mW, it's positive,

I=4mA I'm going to square that and ultimately we end up with a resistance,