。
She maintained a calm attitude throughout.
Literal
She [topic-は] calm attitude [to-に] from-start-to-finish did.
終始する (しゅうしする, 'to maintain throughout, from beginning to end') is a formal suru-verb. 落ち着いた態度 ('calm attitude') uses the past/attributive form of 落ち着く to modify 態度 (たいど, 'attitude'). The sentence has a formal, narrative register.