10. Given the circle $C:(x-1)^{2}+(y-2)^{2}=25$ and the line $l:(2 m+1) x+(m+1) y-7 m-4=0$ intersect, then the equation of $l$ when the chord length intercepted by the circle $C$ is the smallest is $\qquad$ .
10. $2 x-y-5=0$ Hint: It is easy to see that $l$ always passes through the fixed point $A(3,1)$, and $C(1,2)$, when the chord length is minimized, $l \perp A C$.