Can someone me with this problem. i need solving a and b

(a) A generic quadratic expression is often written
  ax² +bx +c
Then the "a" and "b" in the expression -b/(2a) represent the coefficients of the squared term and the linear term, respectively. In your quadratic expression, the variable is "t", not "x", but that makes no difference. You have
  a = -16
  b = 96
Then the time (t) at the maximum height is
  t = -b/(2a)
  t = -96/(2(-16)) = 96/32 = 3

The ball is highest 3 seconds after being fired.

(b) Evaluating h for t=3, we get
  h = -16*3² +96*3 +11
  h = -144 +288 +11
  h = 155

The maximum height of the ball is 155 ft.