The graph of the parabola is represented in the attached diagram.
The vertical axis of symmetry is the line with equation
[tex]x=-\dfrac{b}{2a}=-\dfrac{-12}{2\cdot 3}=2.[/tex]
This axis passes through the parabola vertex with coordinates:
- x=2;
- [tex]y=3\cdot 2^2-12\cdot 2-9=12-24-9=-21.[/tex]
Answer: vertical axis of symmetry is line x=2.
