vertex is (0,5)

line of sym is x=0

take derative of function and equate to 0.

y'= -2x

0= -2x

so x = 0 this is either a min or max. Now you can use the 2nd derative test or plug in values for the 1st derative and see where it increasing or decreasing. For example -1<0<1

y' = -2(-1)= 2

y' = -2(1) = -2

so when x=0 is a max

Hope that helps