y = 43t - 16t2 gives the height of the ball at the time, t, after the ball is released. This is a function about position - in this case, height.
The derivative of the position function yields the function for velocity of the object.
y' = 43 - 32t
The velocity at t = 2: 43 - 32(2) = -21 ft/ sec.
The velocity is negative because the object is slowing down from the original velocity due to the effect of gravity.