Đáp án đúng: C
Phương pháp giải:
+) Sử dụng quy tắc Chuyển vế - đổi dấu.
+) Ta có: \({x^m} = {a^m}\,\) \( \Rightarrow x = a.\)Giải chi tiết:\(\begin{array}{l}390:\left( {5x - 5} \right) = 39\\\,\,\,\,\,\,\,\,\,\,\,\,\,\,5x - 5\,\,\,\, = 390:39\\\,\,\,\,\,\,\,\,\,\,\,\,\,5x - 5\,\,\,\,\, = 10\\\,\,\,\,\,\,\,\,\,\,\,\,\,5x\,\,\,\,\,\,\,\,\,\,\,\,\, = 10 + 5\\\,\,\,\,\,\,\,\,\,\,\,\,\,5x\,\,\,\,\,\,\,\,\,\,\,\,\, = 15\\\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,x\,\,\,\,\,\,\,\,\,\,\,\,\, = 15:5\\\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,x\,\,\,\,\,\,\,\,\,\,\,\,\, = 3\end{array}\)
Vậy \(x = 3\).
Chọn C.