The output voltage $V_{out}$ can be calculated using the formula for an inverting amplifier:
$V_{out} = -V_{in} \frac{R_f}{R_{in}}$
where $R_{in}$ is the input resistance and $R_f$ is the feedback resistance. In this circuit, $R_{in} = R_2 || R_3$ and $R_f = R_1$.
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