I am working through http://mathworld.wolfram.com/Point-LineDistance2-Dimensional.html and have become stuck in a number of places, presently it is in turning step 10 into step 11 and the identity
$|ax_0+by_0+c|=|ax+by-ax_0-by_0|$
I must be missing something from the previous steps. More help may be needed if and when I progress.
Thanks.