[OS X TeX] Aligning equations with comments

[leedwitt at gmail.com]

I need to align x-terms, operator, y-terms, equal sign, and right-hand sides in their own columns, while keeping my comment space to the right. I also need to underline that second equation before writing that third equation.
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I've used hphantom commands to align x and y values before (one equation has 3x, use hphantom{3}x in the next line. Same thing for negatives to align right sides. 

I have no idea about the underlining. 
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