Do \(\left\{{}\begin{matrix}a=1>0\\\dfrac{-b}{2a}=-\dfrac{-2}{2.1}=1< 2\end{matrix}\right.\) \(\Rightarrow\) hàm số đồng biến trên [2; 5]
\(\Rightarrow\max\limits_{\left[2;5\right]}y=y\left(5\right)=5^2-2.5+2m+3=18+2m\)
\(\Rightarrow18+2m=-3\Rightarrow2m=-21\Rightarrow m=\dfrac{-21}{2}\)