。
She admitted that math was her weakest subject.
Literal
She [topic-は] mathematics [subject-が] most weak-at subject is [thing-こと] [object-を] acknowledged.
苦手な科目 means 'weakest/worst subject' — 苦手 (にがて) means 'weak at, not good with.' 最も (もっとも) is the formal superlative: 'most.' ~であることを認めた means 'admitted that it is' — the formal copula である + nominalizer こと + を認める (to acknowledge/admit). A structurally complex sentence with multiple layers.