The Voltage Divider Calculator to find the Vin, Ra, Rb, Vout, when you enter three values then will get the last one.
Vout = R2/(R1+R2) × Vin
Where Vin is Input Voltage, Vout is Output Voltage
For example, when Vin is 4 Volts, Rb is 2 Ohms, Vout is 1 Volts, then the Ra will be 6 Ohms.