、。
She isn't a doctor — she's a teacher.
Literal
She [topic-は] doctor [is-not-で-はなく], teacher is.
AはBではなくCだ ('A is not B but C') is the canonical contrastive identification frame in Japanese. ではなく is the connective form of negative copula ではない — using the い-adjective stem は + なく to link cleanly into the next clause, equivalent to 'not B, (rather) C.' Slightly more careful than the casual じゃなくて. The sentence corrects a possible misidentification while supplying the right answer.