To add regular text to a LibreOffice formula just put it in double quotes. This is useful when inserting mathematical characters into formulas which would otherwise auto-format.
Now, If you want a double quote to appear, it needs to be put in double quotes with a backslash in front. Finally, even though we're getting a bit silly, but if you want a backslash followed by a double quote symbol, just add another backslash.
It helped me to enter *in my equation