Solving Quadratics by the Quadratic Formula

Question

Damian the diver always dives into the deep abyss following the equation d=-5t²+10t+5, where d is the distances above the water (in meters) and t is the time (in seconds). How high is the diving platform? After how many seconds is the diver 8 meters above the water? After how many seconds does the diver enter the water?

-- There are THREE questions in this problem

Answer 1

What's the time for the dive?

Gravity= (t^2+t)/2*9.81m/sec.