A circle in the standard (x,y) coordinate plane has center (3,8) and is tangent to the x-axis. The point (x,y) in on the circle if and only if x and y satisfy which of the follwing equations?

The solution is (x - 3)^2 + (y - 8)^2 = 64. Why do we say the radius is 8? Why couldn't it be 3 with the above information?