Type in the equations as regular formulas, using new names for the variables you want to solve for. The new names should appear at the bottom of the section.

Reference values from outside the section

When you type the equations, you can reference values from the document like normal.

Use the result in equations

When the solver section has enough information to solve for the new variables, the results will appear next to the names at the bottom. Click on these results to use them in formulas.