An astronaut on the moon throws a baseball upward. The astronaut is 6​ ft, 6 in.​ tall, and the initial velocity of the ball is 50 ft per sec. The height s of the ball in feet is given by the equation s=−2.7t2+50t+6.5​, where t is the number of seconds after the ball was thrown. Complete parts a and b. a. After how many seconds is the ball 18 ft above the​ moon's surface? After nothing seconds the ball will be 18 ft above the​ moon's surface.