The ball momentarily stops when the velocity is 0. So set h'(t) = 0 and solve for t to find the time when the ball stops. Then plug the value you got for t into h(t) to get the height.
The height of a soccer ball after it is kicked into the air is given by h(t)=-4.9t^2+3.5t+1, where h is the height in metres and t is the time in seconds
a)determine the rate of change of the height of the soccer ball at time t.
-9.8x+3.5 didnt have problem with that
b) Determine the rate of change of the height of the soccer ball at 0.5s
-9.8(0.5)+3.5= -1.4m/s no problem with that
c) When does the ball momentarily stop?
What is the height of the ball at this time?
...... have no clue......