If you set the x coordinate to , and substitute the x-coordinate part of each of your three conditions, you get three equations of the form . Since the and are known values, you have 3 equations in the three unknowns a, b, and c. Then you do the same thing for the y coordinate.
You might find this simpler than solving 3 equations in 3 unknowns (twice):
Lagrange polynomial - Wikipedia, the free encyclopedia
Post again in this thread if you're still having trouble.