Calculate fillet between arc and a line

I am trying to find some algorithms or formulas for doing a fillet or blend operation on the intersection of an arc and a line segment given a particular radius. In other words, I need to find the equation for the circle that will have tangent points on both the line and the arc.

Eventually, this has to be calculated using VB.Net so any sample code will be appreciated.

as a geometry problem:

1. calculate the line that is parallel to the initial line and r distance away
2. calculate the arc that is parallel to the original arc and r distance away
3. find the intersection of those two items

easier if the two are lines in the first place...