Calculate Vout from an op amp
This circuit is a classic diff-amp.  The output is V2-V1.

One way to analyze this circuit is to think of the affect from each input to the output separately.  Start by grounding V2 and thinking about the response from V1 to the output.  With V2 grounded, the + input is just held at 0.  Now you have a simple inverting amp with a gain of -1 from V1 to Output.

Then ground V1 and see what changing V2 does.  With V1 grounded, Rf and R1 form a voltage divider to make a classic positive gain amp from the + input to the output, with the gain being +2 in this case.  Now note that the two resistors on the V2 input form a voltage divider with a gain of 1/2.  That gain of 1/2 from the voltage divider times the gain of 2 for the amp make a overall gain of 1 from V2 to Output.

So you have a gain of -1 from V1 to Output and +1 from V2 to Output.  Putting these together, you get the overall response of Output = V2 - V1.

For extra credit, figure out how to change the ratio of the resistors to make a diff amp with non-unity gain, like 4 for example.