a. The radius is the distance from the circle's center to the point of tangency:

b. The line tangent to the circle is perpendicular to the line connecting points A and B. Lines perpendicular to one another have slopes that are negative reciprocals of one another. In other words, if the slope of the line through A and B is
, then the slope of the tangent line is
.
Find the slope of the line through A and B:
