Layout > Working with Entities > Modifying Entities > To Merge or Divide Entities
  
To Merge or Divide Entities
1. On the Design tab, click Merge or Divide and do one of the following actions:
For a merge, select two tangent lines, arcs, or splines to merge.
For a division, select a line, arc, or spline to divide.
2. To merge additional tangent lines, arcs, or splines, continue selecting tangent entities of the same type.