Added the script

<script src='https://cdnjs.cloudflare.com/ajax/libs/mathjax/2.7.5/latest.js?config=TeX-MML-AM_CHTML' async></script>

to [/skin/_body_header.inc.php]

Then the text

Where a is not zero.
$$a \ne 0$$ 
$$(ax^2 + bx + c = 0)$$
$$x = {-b \pm \sqrt{b^2-4ac} \over 2a}$$

becomes

Where a is not zero. $$a \ne 0$$ $$ (ax^2 + bx + c = 0)$$ $$x = {-b \pm \sqrt{b^2-4ac} \over 2a}$$