I would direct you to the <a href="http://reference.wolfram.com/language/tutorial/SolvingEquations.html">tutorial</a>, specifically Input No. 10.

To quote

> There is no explicit "closed form" solution for a transcendental equation like this. You can find an approximate numerical solution using FindRoot, and giving a starting value for x.

Like so:

    FindRoot[Sin[x] - x^2, {x, 1}]
>     {x -> 0.876726}