Click the Word menu bar: Insert, Formula and Insert New Formula. In the pop-up formula toolbar, click the label symbol, the second in the third row of the horizontal grid, and enter the letter X in the "Need to pop up" box.

Click the Word menu bar: Insert, Symbol, Other Symbols, select the symbol in the pop-up toolbox, and click Insert. Finally, enter the letter s, and end.

Insert a symbol before lowercase x, that is, symbol insertion, symbol, font selection, symbol. Note: adding a short bar in the upper right corner between underscore _ and alpha can also solve the problem.