Don't let the discussions on STACK in the Mathematics tools forum https://moodle.org/mod/forum/view.php?id=752 distract you. (STACK https://moodle.org/plugins/qtype_stack is an advanced question type for maths.)
You need a simple maths filter like the https://docs.moodle.org/en/TeX_notation_filter or the https://docs.moodle.org/en/MathJax_filter. There are all kinds of graphical interfaces to the GUI (aka Text) editor of Moodle. The text below that looks like this:
P.S. It is a bad idea to copy-and-paste from Word in to the Moodle GUI (aka Text) editor!