A circle 8 cm in radius is tangent to a line at A. Another circle 3 cm in radius is tangent also to the same line at B. Find the distance AB if the two circles are tangent externally to each other.

A circle 8 cm in radius is tangent to a line at A. Another circle 3 cm in radius is tangent also to the same line at B. Find the distance AB if the two circles are tangent externally to each other.