The axis of symmetry of a parabola like
[tex]y=ax^2+bx+c[/tex]
is a vertical line of the form
[tex]x = k[/tex]
where k is the x coordinate of the vertex of the parabola.
In particular, the x coordinate of the vertex of the parabola is given by
[tex]x = -\dfrac{b}{2a}[/tex]
which in your case becomes
[tex]x = -\dfrac{18}{-6} = 3[/tex]
So, the axis of symmetry is the line
[tex]x=3[/tex]
