Please forgive me, but I am having a total mind block..

I am doing a question about a diver, and I have to determine when the diver hits the water. The equation is y= -5t^2 +10t +50.

I have tried simple factoring, complex factoring, and using the quadratic formula and nothing has worked =( Don't know whats going on. I know the answer is 4.3 something through my graphing calculator, but I can't get there.. I have done many questions like this before but I have been sitting here for half an hour trying to figure this one out, and I either did something wrong or I am missing something simple and stupid because I've been doing math all day.

Thanks