Can i just get the work for s=-16t^2+45t+400 i know the answer is 5.211 but idk how they got it

You haven't given us the whole problem.

The equation tells the relationship between  's'  and  't' . 
Now the question has to be either
                                             "Find  's'  at a certain  't' .",
or else it has to be
                                             "Find the  't'  that produces a certain  's'."

You gave us the equation that we'll use to answer the question,
but you never actually gave us the question.

I suspect that the question might be "What is  't'  when  's'  is  200 ?"
But I'm not going to put a lot of work into it until I know for sure.
(Not for 5 points, that's for sure.)