The formula is

h = ho - 16t²

h = the variable height the ball is above the river at variable time t.

ho = the height above the river that the ball is dropped = 150 ft.

When the ball hits the surface of the water is is 0 ft above the river.

So we set h = 0 and ho = 150 and solve for t

0 = 150 - 16t²

16t² = 150

  t² = 

Take the square root of both sides, (we don't need ± because
time is never negative.

   t = 

   t = 

   t = 
  
   t =  or 3.06 seconds approximately.

Edwin