UnmetHours uses MathJax. You surround inline equations by a single $ sign: $e = mc^2$. To put an equation on its own line and centered, use $$

$$ e = m \cdot c^2$$

For the format of the equations, it's like LaTeX. If you are unfamiliar with it, please read this post here (basic) Equation support on unmethours. There are also handy tools such as this online equation editor.

UnmetHours doesn't have support for all of MathJax (on the post I linked to, there are a few things that aren't working in my answer), but the essential stuff is there:

$$\dot m C_p \frac{x}{y} \Delta T$$