The following lines I calculated the output of the amplifier.
Gain=-V1(R1/R2)+V2(R4/R3+R4)(R1+R2/R2)
-3V= -6V(6k/3k) + 5V(3k/2k+5k)(6k+3k/3k)
-3V =-12V + 9V
-3V = -3V
If we increase the output we need to increase the non invertet signal.
If we decrease the invertet singal it has got the same effect. Resistor 6k and 3k are in ratio and describe the gain.
