Perhaps you can use Solve() from the Maxima plugin (this is pre-installed in the portable distribution (see signature). There is an interactive handbook ([?]-Button in the toolbar) with a chapter Mathematics and several pages on equation solving. You also can try the function index and locate the entry for Solve().
Martin Kraska
Pre-configured portable distribution of SMath Studio: https://en.smath.info/wiki/SMath%20with%20Plugins.ashx