I have a op amplifier for which I want to calculate Vout depending on Vin. 

The circuit looks like this:

![](https://i.stack.imgur.com/KaQQ4.jpg)

I know that with a circuit that just has ground connected to + of the amp, i can just calculate KVL: V1 = R1\*I1+Rf\*If+Vout but with this amp, there is also resistors on the plus side of the amp to include in the calculation. I know that I should consider the op to not let any curren through. Therefore I think that I can use KVL on the plus side as well, so that I get two KVL-equations.

My question is now, if I have V1 = R1\*I1+Rf\*If+Vout and V2=R2\*I2+R3\*I3, how do I put them together?

I think that I can assume I1=If and I2=I3, if that helps.