If the "resistors" are real resistors, than you can use U = RxI to determine what voltage will be applied...but if you don't have the resisotrs value (it is often around 56 Ohms), that won't help
Actually, you don't need to know the resistance, you only need to know the ratios. In this case, one resistor gives 7V across the fan and 5V across itself. Two resistors therefore gives (7/(7 + 5 + 5)) * 12 ~= 5V across the fan.
That's assuming the resistors are in series. If they are in parallel then it will be (7/(7 + (5/2))) * 12 ~= 9V