Previous point to another point on a curve. Solved

Helo Everybody.
Anyone know how I can get a previous point on a curve to a given point on that same curve?
I think that is a basic question but i don’t know how to do it.

Thanks in advance

PD: In C# please


        Double value;
        curvaDondeHacerEntrada.ClosestPoint(MyPoint, out value);

        curvaDondeHacerEntrada.PointAt(value -0.1);