Đáp án đúng: A
Giải chi tiết:Ta có:
\(\begin{array}{l}74 + 21:a = 77\\\,\,\,\,\,\,\,\,\,\,21:a = 77 - 74\\\,\,\,\,\,\,\,\,\,\,21:a = 3\\\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,a = 21:3\\\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,a = 7\end{array}\)
Vậy \(a = 7.\)
Chọn A