Do you really have to ask this ? ( Oh i see so in our case one equation is CH3COO- + CH3COOH = original moles of ch3cooh taken ? ) Does one of them magically appear or disappear ?

You have set of equations with the set of variables. It is not a chemistry problem any more, but mathematical one. You have 2 options 1/ Exact solution, that is often more or less complicated 2/ Approximate solution, using conditional strong inequalitities like c >> [H+] >> [OH-] for cases of weak acids.

Let consider weak acid equailibrium: Ka=[H+][OH-]/[HA] ->(if near all H+ comes from HA, i.e. [H+]>>[OH-]), then Ka=[H+][H+]/[HA] -> (if c >> [H+] than near all acid is not disociated ) then Ka=[H+][H+]/c -> [H+]=sqrt(Ka .c ), pH=0.5(pKa - log c )

@Paracetamol When approximate solution is used, it should be tested if simplifying conditions are met.

Be aware that Qs/Aa on SE sites are supposed to fill up holes in knowledge, not laying out foundations. For systematic education textbooks or online educational resources ( libretexts.org is shining example) are much better suited for that,

Before asking, it is expected to: 1) Check for related Q/As by insertion an additional Google search term site:stackexchange.com. 2) Look at other valuable sites using additional search terms like site:libretexts.org or site:hyperphysics.phy-astr.gsu.edu or site:en.wikipedia.org. 3) Read about the topic in available textbooks and search internet-wide.

@Paracetamol 4) If asking, provide summury compact results of 1-3 // See also CH SE resources

Thanks a lot @Poutnik Sir understood i will keep that in mind