Non-Inverting Amplifier:

$V_{out} = V_{in} \cdot \left(1 + \frac{R_f}{R_1}\right)$

Inverting Amplifier:

$V_{out} = -V_{in} \cdot \left(\frac{R_f}{R_1}\right)$

Differential Amplifier:

$V_{out} = V_2 \cdot \left(\frac{R_4}{R_3 + R_4}\right) \cdot \left(\frac{R_1 + R_2}{R_1}\right) - V_1 \cdot \left(\frac{R_2}{R_1}\right)$

If $R_1=R_3$ and $R_2=R_4$, then $V_{out} = \frac{R_2}{R_1}(V_2 - V_1)$