Well the circle equation is

So putting the two points in gives

and

The circle is tangent to the line y = -1. This tells us a few things. First, the circle has a point (X, -1) on it:

We also know that the derivative of the circle equation is 0 at y = -1, since y = -1 is a horizontal line. So taking the derivative of the circle equation we get

The derivative at (X, -1) is 0, so this becomes

Thus you need to solve the system of equations:

-Dan