Since this is a differential amplifier I suppose the first thing I'm suppose to do is to find the voltages at both the inputs of the op-amp, but using KCL doesn't seem to yield enough equations to solve this for me. Any help is appreciated.

 

Suppose that the 1k resistor sits between the input sinusoid (Vi) and V1, and that the second 1k from the top sits between V1 and V2, hence the third 1k sits between V2 and ground.

Let the voltage on the two inputs of the OpAmp be Vx, and let the output be Vo. So, you've got four nodes (V1, V2, Vx, and Vo).

From the Vx node you get two equations, one on the inverting terminal that relates Vo and Vx, and the other on the non-inverting terminal, which relates Vx to V2. You get one equation at node V1, and one at the node V2. This gives you four equations to solve for four unknowns.

Recall that no current can enter into either of the inputs of the OpAmp.