If you have the Symbolic Toolbox, you can try to use solve().
All versions of the Symbolic Toolbox have been able to solve quadratic equations (as far as I know.) In the MATLAB versions new enough to use the MuPAD symbolic engine (about R2008b on) up to about R2010a, a sufficiently complicated cubic or quartic might not have its solution found. R2010b onwards appears to do better on root finding. As per usual, any degree past 4 could be irreducible.
If you are trying to solve transcendental equations (e.g., logs, trig) then the majority of the time there simply is no analytic solution known to mathematics, but solve() will try
