Nếu

- Hướng dẫn giải

Ta có: 

$f^{\text{'}}\left(x\right)=\frac{-1(\text{}3x+1)-3(\text{}2-x)}{{(3x+\text{}1)}^2}=-\frac{7}{{\left(3x+1\right)}^2}$

$\Rightarrow f^{\text{'}\text{'}}\left(x\right)=7.\frac{2\left(3x+1\right).{\left(3x+1\right)}^{\text{'}}}{{\left(3x+1\right)}^4}=\frac{42}{{\left(3x+1\right)}^3}$.

Chọn đáp án C