Calculus · Easy problem

A security camera sits directly above a gate and must tilt down by $\theta(d)=\arctan\!\left(\dfrac{50}{d}\right)$ radians to track a person walking away, a horizontal distance $d$ meters from the base. The camera's motor stalls once the required tilt rate reaches exactly $-\dfrac{1}{250}$ rad/m. Find the distance $d$ at which this happens.