。
She's excellent at writing essays.
Literal
She [topic-は] composition [subject-が] excellent [is-だ].
A characteristic Japanese double-marking pattern: the 'large subject' 彼女は (the topic, the person we're talking about) and the 'small subject' 作文が (the specific area of evaluation) coexist in one clause. The structure literally reads 'as for her, composition is excellent' — telling you both the person and the dimension along which she's being assessed. 作文 ('composition, written essay') is the school-context word for the kind of structured writing students practice; broader writing (literature, articles) uses 文章 or 執筆.