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}