If $x^2+(m+3)x+(m+6)=0$ has equal roots, then the value of $m$ is

$\begin{array}{l}{x}^2+(m+3)x+(m+6)=0\\ a=1,b=m+3,c=m+6\end{array}$

Given roots are equal

$\begin{array}{l}{b}^2-4ac=0\\ {(m+3)}^2-4.1(m+6)=0\\ m^2+9+6m-4m-24=0\\ m^2+2m-15=0\end{array}$

$\begin{array}{l}{m}^2+5m-3m-15=0\\ m(m+5)-3(m+5)=0\\ (m+5)(m-3)=0\\ m=3\text{\hspace{0.17em}\hspace{0.17em}or\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}}-5\end{array}$