\(\Delta'=4-m\ge0\Rightarrow m\le4\)
Theo Viet: \(\left\{{}\begin{matrix}x_1+x_2=4\\x_1x_2=m\end{matrix}\right.\)
\(\left(3x_1+1\right)\left(3x_2+1\right)=4\)
\(\Leftrightarrow9x_1x_2+3\left(x_1+x_2\right)+1=4\)
\(\Leftrightarrow3x_1x_2+x_1+x_2-1=0\)
\(\Leftrightarrow3m+3=0\Rightarrow m=-1\)