In this configuration the OpAmp works as a differential amplifier.
Normally R1=R3 and R2=R4 to facilitate the design and minimize the influence of bias current.
In this case:
Vou = (V+ - V-)*(R4/R3)
In this example R4 is the resistor of 20k and R3 the resistor of 10k.
Ps .: This simulation use DC signals because it's easier to see the numerical results and check with the formula. You can change to sine or square wave if prefer.
