Awoman throws a baseball straight up into the air. what is the baseball's acceleration at its highest point?

After the ball leaves the woman's hand, the only relevant force acting on the ball is the gravity, which produces an acceleration of a=g=9.81 m/s^2 on the baseball, directed downward. 
Gravity acts on the ball for the whole duration of the motion, so the acceleration of the ball is  at every point of the motion, and therefore also when the baseball is at its highest point.

So since the snail is moving at 3.6  miles per hour you multiply by 5280 to change it into feet per hour. so you get 36*528 ft per hour. since there are 3600 seconds in an hour, you divide 36*528 by 3600. so simplifying we get 528/100 which is 5.28 ft per second