An inverting summer holds the inverting input at virtual ground, so each input resistor contributes an independent current and the feedback resistor converts their sum into an inverted output voltage. The 2 V and 4 V inputs through 2 ohm and 4 ohm give 1 A plus 1 A, and the 3 ohm feedback yields -6 V exactly for the ideal op-amp model; real summers add input bias and offset, finite open-loop gain and bandwidth, resistor tolerance, rail limits, source impedance, loading, and measurement uncertainty.
An inverting summer adds exact input currents before feedback converts them to voltage. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.
highlighted = computed this step
Each input resistor contributes a current
The first input contributes 1 ampere, and the second contributes 1 ampere.
Ia+Ib=1A+1A
The feedback resistor converts total current to output
The total current of 2 amperes through 3 ohms gives -6 volts.