。
She accompanied the violin solo on the piano.
Literal
She [topic-は] piano [with-で] violin solo [of-の] accompaniment [object-を] did.
独奏 ('solo, solo performance') is the Sino-Japanese term for a single instrumentalist's performance, made of 独 ('alone, single') + 奏 ('play, perform, music'). The closely related 独唱 ('solo singing') swaps in 唱 ('sing'). When a soloist needs piano backing, that backing is 伴奏 ('accompaniment'), so the phrase バイオリン独奏の伴奏 reads as 'the accompaniment to a violin solo,' with の linking the head 伴奏 to its modifier 独奏.