Op-amp amplifier feedback, offset
The resistor R3 is not only a part of the feedback network (in this case it would only influence the gain) but is is connected to a second input voltage (10 volts). Hence, it influences the gain and - in addition - is part of an amplifier that is connected to TWO input voltages (AC signal at the non-inv. input and a DC voltage at the inv. input) 

For this purpose you can redraw the circuit and use another 10 volt dc source as a second input voltage (connected to R3)...this clears the situation and enables you to see that you have, indeed, a difference amplifier (both input nodes of the opamp receive an input signal - one is ac and on is dc).

Now remember that in such a case you can use the superposition theorem for splitting the calculation in two separate parts.