Currently Math does not have an alignment construction. Instead you can use a matrix.

For example to typeset the equation

you should enter the following:

matrix{
 alignr x+y # {}={} # alignl 2 ##
 alignr x   # {}={} # alignl 2-y
}

The empty braces around = are necessary because = is a binary operator and thus needs an expression on each side.

You can reduce the spacing around = if you change the inter-column spacing of the matrix:

Here is another example using a different solution:

To typeset the equation

the trick is to use phantom as follows:

""3(x+4)-2(x-1)=3 x+12-(2 x-2) newline
""phantom {3(x+4)-2(x-1)}=3 x+12-2 x+2 newline
""phantom {3(x+4)-2(x-1)}=x+14

The effect of the empty quotation marks is to left justify the line - a line starting with text (even an empty text) will always be left justified.

You can replace "" with alignl.