Schematic

Current through a resistor is given by V/R, where V is the voltage across
the resistor and R its resistance. In the diagram above, use a 1000 Ohm
resistor as R2 so that the current can be calculated easily, it is just the
voltage at A1 divided by 1000.
If R1 is unknown, we can calculate it from (PV1-A1)/I.