Now use some geometry: draw a circle and a point exterior to it and draw:
== the line joining the point and the circle's center and
== the point and one of the two tangency point on the circle (this is just one of the two tangents determined by the point and the circle) and
== the radius to the tangency point chosen.
You get a straight angle triangle whose straight angle is between the tangent and the radius (this is a basic theorem in geometry) ==> the distance you're looking for is the length of the catetus formed by the point and the tangency point ==> by Pythagoras theorem, this distance squared equals the square distance between the point and the circle's center minus the square of the radius.
Well, now just do the maths.