Thevenin equivalent of the input of an op amp?
There is a pretty much general method for finding the Thevenin equivalent circuit:


Find the open circuit output voltage. This means take the circuit you are trying to find the equivalent of, replace the load with an open circuit, and find the output voltage.

In your circuit this means replacing everything to the right of node "B" with an open circuit. So the output voltage is given by the voltage divider rule,
$$V_{oc} = v_s\frac{R_2}{R_1+R_2}$$
Find the short circuit output current. In your circuit this is given by
$$I_{ss} = \frac{v_s}{R_1}$$


Now the Thevenin source voltage is \$V_{oc}\$ and the Thevenin resistance is \$\frac{V_{oc}}{I_{ss}}\$.