The time to complete one jump is the difference in the values of t for which h(t) = 0. A graphing calculator shows this nicely, but you can also figure it out algebraically.
Since the quadratic has no constant term, it is easily factored.
0 = -16t^2 + 28t
0 = -4t(4t -7)
This product will be zero when one or both factors are zero.
For -4t = 0
t = 0
For (4t -7) = 0
4t = 7
t = 7/4 = 1.75
This tells us the dolphin leaves the water at t=0 and completes the jump at t=1.75. The difference between these times is 1.75 - 0 = 1.75 seconds.
The length of time in seconds that it took the dolphin to complete one jump was
1.75