Because there is no formula editor installed in your word, if you use word2003, you don't need to reinstall it.

You can click Tools on the toolbar of word, click Customize, click Command, find Insert in the box of the drop-down menu on the left, then find the pattern of Formula Editor with the root number A in the box on the right, select it with the left key, and drag it to the back of other tool patterns such as Color and Font in the toolbar of word. Double-click it to enter such a symbol.

If other versions of word are installed, some will be reinstalled.