Op-amp voltage calculation
I hope you can understand my point.


Not really.







simulate this circuit &ndash; Schematic created using CircuitLab

Figure 1. The output and voltage divider.

Your calculation is correct but the -6 V appears at the op-amp output. The 10k and 40k are a potential divider as you correctly point out and your calculation is correct.

Your second diagram is showing the divider and \$ V_O \$. When doing the calculations we need to assign a direction for the voltage calculation on the diagram. In your example you have chosen the bottom of the 40k as the negative for the measurement and the top as +. The sign of the calculation result indicates that, actually, the top of the 40k resistor is at a lower voltage than the bottom. Everything is mathematically consistent. If you connect your multimeter black lead (COM) to the bottom of the 40k resistor and the red (V) lead to the top the meter would display -4.8 V.