Consider the curve given by the parametric equations

$$x = t (t^2-12) , \quad y = 6 (t^2-12)$$
a.) Determine the point on the curve where the tangent is horizontal.
$t=$

b.) Determine the points $t_1$, $t_2$ where the tangent is vertical and $t_1 < t_2$ .

$t_1=$
$t_2=$