。
She decided not to attend the meeting.
Literal
She [topic-は] meeting [in-には] not-attend [nominalizer-こと] [object-を] decided.
~ことを決める is the standard 'decide to V' construction: the verbal phrase 出席しない ('not attend') is nominalized with こと, then taken as the object of 決める ('decide'). The contrastive には (に + は) on 会議 hints at a contrast — perhaps she's attending other things, but not this one. 出席する is the formal-leaning verb for attending events, more deliberate-sounding than the everyday 出る. The whole sentence has the calm, deliberate tone of a recorded decision.