mirror position across vector

How can i calculate the mirrored position for a point? Keep in mind these are points I'm dealing with, NOT transforms. So the only information i have is positions. The reason i mention position only, because ill be doing this on the vertex level.

You can see in the image I want to mirror the one point across the vector created by the other two points. This is in theory only 2D space, x,y.

Hope someone can help. Thanks

delete objects
 
a = point pos:[5,5,0] size:.5 wirecolor:yellow
b = point pos:[0,5,0] size:.5 wirecolor:red
c = point pos:[5,0,0] size:.5 wirecolor:red