Question 762753
Use the formula x = -b/(2a) to find the x coordinate of the vertex


x = -b/(2a)


x = -10/(2*(-1))


x = -10/(-2)


x = 5



The x coordinate of the vertex is at x = 5


Plug it in to find the max y value


f(x) = -x^2+10x+9


f(5) = -(5)^2+10(5)+9


f(5) = -25+10(5)+9


f(5) = -25+50+9


f(5) = 25+9


f(5) = 34


So the maximum y value is y = <font color="red">34</font>