hi,
The gain relative to the -Vin of the OPA is R1/R2= 2, the gain relative to the +Vin of the OPA is 1+[R1/R2]=3.
With the R3,4,5 you have 1/3 division so effectively the the OPA gain relative to the S1 and S2 is 1.
The divider R6 and R7 gives a Vref of 5V* [330/1333]=1.25V, so the -Gain of 2 gives an offset of -2.5Vout, which is cancelled by the +2.5V due to 2.5V of S2, ie= 0 Vout
I use the low output impedance of the Vref OPA in order to to ensure the gains of the 'main' OPA are correct when using those resistor values for the gain.
E.