Use this circuit diagram to show how the op-amp will detect an imbalance in the circuit and state what outputs it would give.                                       

If the voltage in between R1 and R2 > than voltage between R3 and R4 (P)

Output will be negative (P)

If the voltage in between R1 and R2 < than voltage between R3 and R4 (P)

Output will be positive (P)