Hi, apply the formula:
Xv = - b / 2a
Where,
b = 176
a = -16
c = 20
Then, the value of Xv will be:
Xv = - (176 / 2 .-16)
Xv = 176 / 32
Xv = 5,5 s
So , just us will go to make the substitute of Xv in equation.
And we will go to find the value of H
H(t) = Yv
As , Yv = h(Xv)
H(t) = -16t^2 + 176t +20
Yv = - 16.(5,5)^2 + 176.(5,5)+20
Yv = -484 + 968 + 20
Yv = 504 feet
This would be the answer to the your question.
Hope this helps