Solve an applied problem using a quadratic function.

A Martian standing on top of a boulder has tossed a rock vertically upward. The quadratic function below models the height of the rock, $h(t),$ in feet, $t$ seconds after it was thrown.

$h(t)=-6t^2+18t+48$

How long will it take for the rock to hit the surface of Mars? $($Round your answer to the nearest tenth.$)$