Chọn C.
\(\begin{array}{l} {\int _{1}^{2}\left[3f\left(x\right)-2x\right]dx =6\Leftrightarrow 3\int _{1}^{2}f\left(x\right)dx-2\int _{1}^{2}xdx =6\Leftrightarrow 3\int _{1}^{2}f\left(x\right)dx-2. \left. \frac{x^{2} }{2} \right|_{1}^{2} =6} \\ {\Leftrightarrow 3\int _{1}^{2}f\left(x\right)dx=9\Leftrightarrow \int _{1}^{2}f\left(x\right)dx=3 .} \end{array}\)