If the circle just "touches" the x-axis, you know that the radius of said circle will be 5. From that, you should be able to calculate A and B (use the standard form of a circle, and set y to 0). Remember that a tangent is perpendicular to the line that forms from the center of the circle to the point of tangency. Find the line perpendicular to that radius, and find where it intersects the y-axis.