The vertex form:
[tex]y=a(x-h)^2+k\\\\(h,\ k)-vertex[/tex]
The axis of symetry is x = h.
[tex]y=ax^2+bx+c\to h=\dfrac{-b}{2a}[/tex]
We have
[tex]f(x)=-3x^2+18x-7\to a=-3,\ b=18,\ c=-7[/tex]
Substitute:
[tex]h=\dfrac{-18}{(2)(-3)}=\dfrac{-18}{-6}=3[/tex]
Answer: x = 3
