Hi,
There are some different approaches.
The simplest method (for me) is.
* it is a difference amplifier (therefore the difference input voltage is: V2 - V1
* it's output is GND referenced (no need to add a constant voltage to the output)
* gain is : R4 / (R4 + R3)
Therefore Vo = ( V2 - V1) × ( R4 / (R3 + R4))
For random Opamp circuit calculations:
* first calculate the voltage at V+ input
* then decide that V- input has the same voltage, as V+, no Opamp input current
* with this information you can calculate the output voltage
Klaus