"Falar com segurança" can also be translated to portuguese as " To speak with assurance".

                 Noun 1. assurance - freedom from doubt; belief in yourself and your abilities; "his assurance in his superiority did not make him popular"; "after that failure he lost his confidence"; "she spoke with authority"
confidence, self-assurance, self-confidence, sureness, authority
certainty - the state of being certain; "his certainty reassured the others"
               Fonte/Source http://www.thefreedictionary.com/assurance