LaTeX.org

MathJax is not LaTeX! It is a JavaScript library to display math in a similar language, but it is not the same. It is not TeX based. And therefore it is mostly off-topic here.

However, maybe you can use \matrix or \array (two examples, where LaTeX would provide an environment instead of a command ...

How do I enter the calculation below in MathJax (Math verison of Latex)? I can enter the Latex with or without the document capabilities, but it's easier with just the Math part of Latex.

First Step:
54 -> (50 + 4)
x7
/hline

Second Step:
50 x 7 = 350
4 x 7 = +28
\hline
+378