You can use MathJax on your Postach.io website as the example below:

When $a != 0$, there are two solutions to $ax^2 + bx + c = 0$ and they are:

Another example:

When $x < y$ or $x > y$ or $x^2$ or $a_(m n)$ or $a_{m n}$ or $(x+1)/y$ or $sqrtx$ we will have the following result:

The examples above has the following code: