Dan, like always - there are some different ways to solve the task.

That means, you can start at "zero" with equations derived from basic laws (KVL and KCL) or you can start - for example - with the classical equation for feedback.

I prefer this way because it is straight-forward and relatively simple (and logical).

1.) The basic formula for the closed-loop gain of a circuit with feedback is **Acl=Hf*Aol/(1-Aol*Hr) = Hf/(1/Aol - Hr)**.

Aol: Open-loop gain of the opamp, Hr: Feedback factor (sometimes also called "beta", Hf: Forward damping factor.

2.) __Definitions: __

Hf=(Vp-Vn) for Vout=0 and Hr=(Vp-Vn) for Vin=0. (Vp, Vn: voltages at the pos. resp. negative opamp input nodes).

For negative feedback (usual case) this leads to a __negative__ feedback factor Hr.

3.) Assumng Aol to be infinite, we have: **Acl=-Hf/Hr**

4.) Now, you can separately compute Hf and Hr (simple voltage divider rules) and find the ratio.

This method has the advantage that you have divided the whole calculation task in two separate steps.

Try also to find the solution for three equal resistors. It is the transfer function for a well-known filter circuit.