The Solver add-in is included by default in Microsoft Excel. You only need to activate it to load it. Learn how to load Solver add-in in Excel if it is not showing.

Excel can solve for unknown variables, either for a single cell with Goal Seek or multiple cells with Solver.

Excel can solve equations through several of its mathematical functions, but the single Excel tool that works on the largest variety of equations is the program's Solver Add-in.