How to solve this problem in the simplest way via LTspice?
I think you can check it with LTSpice but, to &quot;solve it&quot; I'd use Millman's theorem: -


Above pictures from this website.
This gets you the voltage at the mixing point (the non-inverting input to the op-amp). To add gain is simply to use an op-amp very simply. So, choose your input resistor values to suit and add gain to bring the output inline with what you require.