Calculating the transfer function of this op-amp circuit
Here's how I would start it: -



You can calculate the filter TF seperately but concentrate on solving the feedback problem first with the unknown TF as "TF": -



Write down what you know: -


V1 = Vin - V2
V2 = V1.TF


From (2) V1 = V2/TF and from (1) V2/TF = Vin - V2

Solve for V2 using (1): -

V2(1 + 1/TF) = Vin therefore

\$\dfrac{V_{OUT}}{V_{IN}} = \dfrac{1}{1+TF}\$

Then solve the transfer function TF (it looks fairly easy given that it is a sallen key 2nd order preceded by a simple 1st order filter with a buffer between them hence no impedance interactions).

Sallen key stage (help from wiki): -



That's the hard bit and H(s) just multiplies with the transfer function of the low pass filter which I'll leave you to do.

Then you'll have "TF" which you can insert into the equation I derived.