Answer:
see the explanation
Step-by-step explanation:
we have the quadratic equation

This is a vertical parabola open downward
The vertex is a maximum
Find the x-intercepts of the quadratic equation
The x-intercepts are the values of x when the value of y is equal to zero
so
For y=0

Solve the quadratic equation
The formula to solve a quadratic equation of the form
is equal to

in this problem we have
so

substitute in the formula





therefore
This parabola has two x-intercepts representing the times when the dolphin's height above water is zero feet