Question 1188443
.
A stone is tossed from a bridge into the river below. Its path is represented by h = -2t^2 + 7t + 9, 
where h is height, in metres, and t is time, in seconds.
Find the maximum height reached by the stone.
How many seconds does it take to reach the maximum height?
How many seconds does it take for the stone to reach the water?
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The equation in your post,  describing the height as a function of time,   I S   I N C O R R E C T.


The correct equation is  TOTALLY  DIFFERENT.



How this equation is presented in the post, it makes it clear that the person who created / composed this problem, 

is mathematically  ILLITERATE  and unfamiliar with the corresponding sections of  Math and  Physics.


So,  solving this problem,  in the form as it is presented in the post,  is  NONSENSE.



Due to this reason,  the response by @MathLover1 is  NONSENSE  and  BAD  TEACHING,  too.



To learn the subject,  read the lessons

&nbsp;&nbsp;&nbsp;&nbsp;- <A HREF=https://www.algebra.com/algebra/homework/word/travel/Introductory-lesson-on-a-projectile-thrown-shot-launched-vertically-up.lesson>Introductory lesson on a projectile thrown-shot-launched vertically up</A> 

&nbsp;&nbsp;&nbsp;&nbsp;- <A HREF=https://www.algebra.com/algebra/homework/word/travel/Problem-on-a-projectile-moving-vertically.lesson>Problem on a projectile moving vertically up and down</A>

&nbsp;&nbsp;&nbsp;&nbsp;- <A HREF=https://www.algebra.com/algebra/homework/word/travel/Problem-on-a-toy-rocket-launched-vertically-up--from-the-top-of-a-platform.lesson>Problem on a toy rocket launched vertically up from a tall platform</A>


Consider these lessons as your textbook, &nbsp;handbook, &nbsp;tutorials and &nbsp;(free of charge) &nbsp;home teacher.
Read them attentively and learn how to solve this type of problems once and for all.


Start reading from the very first lesson.