Đáp án đúng: C
Giải chi tiết:\(\begin{array}{l}\,\,\,34 + \left( {21 - x} \right) = \left( {3747 - 30} \right) - 3746\\\,\,\,\,\,\,\,\,34 + \left( {21 - x} \right) = 3717 - 3746\\\,\,\,\,\,\,\,\,\,34 + \left( {21 - x} \right) = - 29\\\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,21 - x\,\,\,\,\, = - 29 - 34\\\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,21 - x\,\,\,\,\, = - 63\\\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,x\,\,\,\, = 21 - \left( { - 63}\right)\\\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,x\,\,\,\,\,= 21 + 63\\\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,x\,\,\,\,\, = 84.\end{array}\)
Chọn C.