Can you find one of two resistances in a series circuit from the voltage, one resistance, and the voltage across the unknown resistor?
If it's a series circuit then "one is serving as a current source" is not true. If it were then you'd need only know what that current is and use V=IR across the unknown resistor.

If you don't know the current, which is more likely, then you can still determine the resistance.

I'm assuming the circuit looks like this:







simulate this circuit &ndash; Schematic created using CircuitLab

Start with what you know:

Vin (known) = Vr1 (unknown) + Vr2 (known)


Vr1 is now known.

Vr1 (known) = Ir1 (unknown) * R1 (known)


Ir1 is now known.

Ir2 (unknown) = Ir1 (known)


Ir2 is now known.

Vr2 (known) = Ir2 (known) * R2 (unknown)


R2 is now known and you are done.