Objective
Find all voltages (and currents) in an electronic circuit that contains operational amplifiers.
Procedure
Verify that the following conditions are met:

Negative feedback circuit

Ideal operational amplifier
Add the following “rules” to Node Voltage Analysis:

Write a KCL equation at both input nodes (V^{+}, V^{}) of the opamp (except if the voltage is known, e.g. set by a source)

Do not write an equation for the output node of the opamp (V_{out}).

Add V^{+} = V^{} to the equations to solve the circuit.
Example: Differencing Amplifier
Let’s use NVA to determine V_{out} as a function of V_{in+} and V_{in} (ignore the part number, the circuit works with any opamp).
Write KCL equations for nodes V_{in+} and V_{in} (steps 5 to 7 have been combined for brefity). No equations are needed for V_{out} (NVA opamp analysis rule) or the inputs V_{in+} and V_{in}.
Solve the equations to get
The circuit amplifies the difference of the input voltage, a very useful function for example with bridge circuits.
Reference: Schaum’s Outline of Electric Circuits, Seventh Edition, Section 5.4.