A person standing close to the edge on top of a \( 28 \) -foot building throws a ball vertically upward. The quadratic function \( h = - 16 t ^ { 2 } + 108 t + 28 \) models the ball's height above the ground, \( h \) , in feet, \( t \) seconds after it was thrown.
a) What is the maximum height of the ball?
b) How many seconds does it take until the ball hits the ground?