This must be a series circuit we're talking about. If it is, then the current is the same through all resistances in the circuit, therefore if you find the current through one of the resistors, you can find the voltage drops across each of the rest.
Once you have the voltages, simply add them to resolve the supply voltage.
In your example, the current through the circuit is 0.25 Amps (2/8).
The voltage across R1 is therefore 4 Volts (0.25 *16) and the voltage across R3 is 0.5 Volts (0.25*2).
The supply voltage for the circuit is 6.5 Volts (VR1+VR2+VR3).
The formulas I used are I=V/R and V=IR.
Kirchoff's Voltage law is the basis for the calculations. It states:
The sum of all voltage drops and rises in a closed loop equals zero.
So the voltage drops across the resistors must be equal and opposite to the supply voltage.