Try and do your math with pure integer arithmetic. This means your functions getDeterminant, getIntersectionX etc. should return an object (or similar) representing numerator and denominator as integers, rather than doing the divisions. Then you will know whether you have a geometry bug or a rounding error - and in the latter case you will win the kata :)

This kata needs harder test cases: non-convex polygons, and/or points aligned with one of the vertices.