I am trying to solve this question, please look at the attached picture
The general equation for a inverting amp is -Rf/R1 * Vin = Vout
The Attempt at a Solution
Well as the question says the two resistors, R2 and R must be treated as parallel so I think the best way to do it would be to use the three resistors, R1, R2, R to find an equivalent resistance for R1. I thought I could do this by thinking that R1 is in series with the parallel combination on R2 and R but when I do the calculation and use the general equation I get the wrong answer which is 20k Ohms . Can anyone offer a hand?