How to determine the gain of this op amp
Each of the amplifiers is driven by a low impedance source so all you have to do is write an equation for the output voltage of each amplifier as a function of the one or two input voltages and then combine them.

You have to combine them because the voltage is fed back in a loop. The net feedback is negative (3 inverting amplifiers in a loop). 

The particular op-amp in question you can equate the input voltages, which for an ideal op-amp means that the non-inverting input and inverting input are both at 0V, so Vx = -(V1 + Vy) where Vx is the output voltage of the left bottom op-amp and Vy is the output voltage of the top op-amp. You should label the voltages at the nodes, and it would have been good to do that before asking the question.