Voltage divider: Vin - R1 -Vout-R2- Gnd
Vout = Vin R2/(R1+R2)
Do some rearranging:
Vout(R1+R2) = VinR2
VoutR1 + Vout R2 = VinR2
VoutR1 = VinR2 - VoutR2
VoutR1 = (Vin - Vout)R2
VoutR1/(Vin - Vout) = R2
Ex: Vout 3V, Vin5V, R1=10K
3*10000/(5-3) = 15000
check Vout = Vin R2/(R1+R2)
Vout = 515000/(10000+15000) = 3V