A particle moves along a path described by parametric equations $x(t) = 12t - t^2$ and $y(t) = 9t - 0.5t^2$, where $t$ is time in seconds. The particle's speed at time $t$ is $v(t) = \sqrt{(\frac{dx}{dt})^2 + (\frac{dy}{dt})^2}$. At what time is the speed zero?