Q.Determine the output voltage when $V_1 = -V_2 = -1\text{ V}$.

The difference amplifier, or subtractor, produces an output proportional to the difference of two independent input signals. One signal $V_1$ is applied through a resistor to the inverting $(-)$ input with feedback resistor $R_f$; the other signal $V_2$ is applied through a resistor to the non-inverting $(+)$ input, which is also returned to ground through a resistor.

The output is found by superposition — treating each input's effect separately (the inverting input acts as an inverting amplifier, the non-inverting input acts through the divider and non-inverting gain) and adding the results:

When the resistor pairs are matched (the inverting side $R_1=R_f$ and the non-inverting side $R_2=R_3$), this collapses to the clean, symmetric result:

$V_o=V_2-V_1$ …