How to adjust alignment within Math ($$)?

[Ethan_Pixar]

I tried using “&&” as in the document. It works for most of the case, but when the equations are inside the cases function, then “&&” lost its function, like these:

== 1.3

$ cases(
     2y_1 +&&  y_2 =&&    4x_1\
      y_1 +&& 2y_2 =&&  -12x_2\
    -2y_1 +&& 3y_2 =&&     x_3
) $

$
     2y_1 +&&  y_2 =&&    4x_1\
      y_1 +&& 2y_2 =&&  -12x_2\
    -2y_1 +&& 3y_2 =&&     x_3
$

These will give: