"align" in whatever software this site uses instead of MathJax

align works for me:

\begin{align} R^2 &= \frac{\sum(\text{fitted value minus sample mean value})^2}{\sum(\text{observed value minus sample mean value})^2} \\ & = \frac{\text{explained variability}}{\text{total variability}}. \end{align}

I'm pretty sure Codidact uses MathJax. We have a Meta tag named "mathjax-rendering".