Kirchhoff's second law
Two batteries and to resistors make an closed circuit. Change voltages of the batters and the resistances of the resistors. See how the voltage behaves around the circuit.
Set one battery to 1.5 V and the other to 4.5 V. Set the resistances to 5 ohms and 2 ohms. Where are the voltage drops and how big are they? What is the current in the circuit?