Thevenin equivalent and op amp
First deal with the op amp. It has a gain = -Rf/R1 so that the voltage feeding into R2 is -vi * Rf/R1.Note that the op amp will invert the signal voltage. The output resistance is taken as zero for a perfect op amp.

C is in parallel with R3. This needs to be reduced to a single impedance.


And at that point you should be able to easily reduce the circuit to a Thevanin equivalent.