Home tab > Draw group > 
Arc menu > Tangential line between two circles/arcs, JOINTAN
Draw a tangential line between two circles or arcs.
Do the following:
-
Select the first object on the side that you want the tangential line to connect to.
-
Select the second object on the side that you want the tangential line to connect to.
The program calculates the shortest path between the objects and draws the tangential line according to that.