solved: mathjax align too much space between &
when you need to shorten empty space betwen begin{array}, (the form can be array, align, matrix, cases, etc)use \kern 000pt between & and The-fixed-point\begin{array}{r l}X &= 2 \times 6 \\&= 12\end{array}>>>\begin{array}{r l}X & \kern -7pt = 2 \times 6 \\&\kern -7pt = 12\end{array}