Please help me with this math problem!!?

I got this question as part of an assignment and I can't for the life of me figure it out! Could somebody please help me out?

A projectile is thrown upward so that its distance above the ground after t seconds is given by the function h(t) = - 16t² + 640t. After how many seconds does the projectile take to reach its maximum height?

I made the function look more normal to understand it better: f(x) = - 16x² + 640x
Please show your work, if possible, so I can understand how it's done.