Calligra Sheets can be used to solve algebraic expressions such as x + x^2 = 4 or For what value of x does x + x squared equal 4 ?

For this example you could enter =A2+A2*A2 into A1 then either try different values in A2 until the result in A1 is as close as you wish to 4 or, preferably, use Calligra Sheets's Goal Seek... feature which automatically adjusts the value in one cell to try to make the value in another cell as close as possible to a target value.

It is invoked by selecting Goal Seek... from the Data menu. This brings up a dialog box in which you should enter the reference of the target value cell (A1 in this case) into the Set cell: box, the target value itself (4) into the To value: box and the reference of the cell that is to be changed (A2) into the By changing cell: box. Note that you need to have entered some initial value into the cell that is to be changed before starting Goal Seek.

Pressing the OK button will start the calculation. When it finishes and if it has found a solution press the OK button to accept the result or Cancel to keep the original value.