WordPress does not support Latex out of the box. You have to install a plugin to write mathematical equation like Calculous. Here are the steps to enable latex support-

1. Install the “Mathjax-Latex” plugin from WordPres.org. You can either search the name from your WordPress admin dashboard or download the plugin directly from here https://wordpress.org/plugins/mathjax-latex/
2. After you have activated the plugin, you can write a mathematical equation anywhere on your WordPress website. You just have to wrap the markup with [latex] [\latex]. In some cases the [latex] shortcode might not work. In that case, you might need to use the alternatives such as $$...$$ and $$...$$)

Here is what I have written on the editor

Here is how the output looks like

You can also use the mathematical expressions in Quizzes as well. Please note that the quiz question does not have custom HTML support. So, you will have to use the question description area to write the latex code. The answers support custom HTML. So, you can write the latex code directly there.

If you have no idea about how to use latex, you can use the guide hosted at the Edinburg University Website- http://www.docs.is.ed.ac.uk/skills/documents/3722/3722-2014.pdf