6/(x2-x-2)=6/(x2-x+¼-¼-2)=6/((x-½)2-2¼).
Let X=x-½, then the expression becomes f(X)=6/(X2-2¼).
In this expression (-X)2=X2, so f(X)=f(-X), so it is an even function with an axis of symmetry at X=0.
Therefore the given expression has an axis of symmetry at x-½=0, that is, x=½. The left and right halves of the curve are reflections of one another.