Source : Webster's Revised Unabridged Dictionary (1913)

Assertive \As*sert"ive\, a.
   Positive; affirming confidently; affirmative; peremptory.

         In a confident and assertive form.       --Glanvill.
   {As*sert"ive*ly}, adv. -- {As*sert"ive*ness}, n.

Source : WordNet®

assertively
     adv : in an assertive manner; "`I will take care of my own life,'
           she said assertively" [ant: {unassertively}]