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left or right in coordinate space

Hi all.

Im new to the forum, and came across it on the hunt for some maths brains better than mine. I'm a hobbyist programmer and have hit a barrier. Eternally grateful if you can help.

In the diagram there are two points creating a line in a coordinate space. This is for Flash, so the origins of x and y are in the top left corner, but hopefully this shoudn't matter. When point B moves, I get a third point (C). I need a reliable method for determining whether point B has moved left or right of the original line. All I know are the coordinates for the two orginal points and the new position of A.

I'm happy with finding distances between points, angles etc - it's the application in this context I'm finding tricky.

If anyone has an idea of where might I start with this, I'd be grateful.

Many thanks.