Pete and Nick are pilots for the US Navy. They are being trained to fly planes which are launched from an aircraft carrier. Their instructor showed them the following graph, which represents the flight path Pete and Nick should take once their plane leaves the deck of the ship; it is a graph of the function

a(t) = t2 - 6t + 24,



where t represents the amount of time in the air in seconds, and a(t) represents the plane's altitude in feet (0 feet represents sea level). The deck of the carrier is 24 feet above sea level. After leaving the deck, at what altitude will the plane be closest to sea level?