Initial velocity is v(0), which is 50
The maximum height of the rock occurs when the velocity is 0. (Think about it, that is the time when the rock is neither moving up nor down)
at max height
Now you know when the maximum occurs, at t = 5
The initial position is 100. So s(0) = C = 100
Maximum = s(5)
So the answer is 225m.
Hope that was helpful