\(\Delta=25-4\left(3m-1\right)>0\Rightarrow m< \dfrac{29}{12}\)
Theo hệ thức Viet: \(\left\{{}\begin{matrix}x_1+x_2=-5\\x_1x_2=3m-1\end{matrix}\right.\)
\(x_1^3+x_2^3+3x_1x_2=75\)
\(\Leftrightarrow\left(x_1+x_2\right)^3-3x_1x_2\left(x_1+x_2\right)+3x_1x_2=75\)
\(\Leftrightarrow\left(-5\right)^3+15\left(3m-1\right)+3\left(3m-1\right)=75\)
\(\Leftrightarrow m=...\)