Negative feedback ensures the voltage across R1 is equal to the input voltage.
R1 forms a potential divider with R2.
Look at the schematic below:
**broken link removed**
[latex]V_\mathrm{out} = \frac{R_2}{R_1+R_2} \cdot V_\mathrm{in} [/latex]
Compare it to R1 and R2 on your schematic.
R1 and R2 swap places.
Vin becomes Vout
Vout becomes V(R1).
Draw the above schematic with the modifications described above.
Change the symbols in potential divider formula to match your circuit.
Rearrange so Vin (on the old formula) or Vout on the new formula becomes the subject and you'll have a fomula that should look like the gain equation for your op-amp circuit.