Ответ: Did she break her leg yesterday? No, she didn't. She broke her arm.
Составляем вопрос, используя вспомогательный глагол Did для прошедшего времени (Past Simple) и информацию из условия: break her leg / yesterday.
Did she break her leg yesterday?
Отвечаем отрицательно, используя no/her arm из условия. Cоставляем отрицательный ответ: No, she didn't. She broke her arm.
Ответ: Did she break her leg yesterday? No, she didn't. She broke her arm.