Answer:
You can find the answer by using the formula t=-b/2a
Explanation:
Remember the maximum height will be at the vertex. The x value of the vertex is your time, so use x=-b/2a except that t is your x. Then if it had also asked what the height was, you would plug that answer into your equation to find the y value of the vertex. Pretty sure your teacher is just asking you to find the x though:)