Lời giải:\(\begin{array}{l}\,C\left( x \right) - 2.B\left( x \right) = A\left( x \right)\\ \Rightarrow C\left( x \right) = A\left( x \right) + 2.B\left( x \right)\\\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\, = 3{x^3} + 3{x^2} + 2x - 1 + 2.\left( {3{x^3} - 2{x^2} + x + 4} \right)\\\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\, = 3{x^3} + 3{x^2} + 2x - 1 + \,6{x^3} - 4{x^2} + 2x + 8\\\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\, = \,\left( {3{x^3} + 6{x^3}} \right) + \left( {3{x^2} - 4{x^2}} \right) + \left( {2x + 2x} \right) + \left( {8 - 1} \right)\\\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\, = \,\,9{x^3} - {x^2} + 4x + 7\end{array}\)
Chọn D
