AAARGH, I found another one.
[y1(x2-x2) was changed to y1(x3-x2)]
y1(x3-x2)+y2(x1-x3)+y3(x2-x1)
a=(-1)-----------------------------
(x3-x2)(x1-x3)(x2-x1)
Okay, do that Gauss elimination for yourself
if you don't believe me after this. :)
PG