Explanation
Our equation
[tex]y=\frac{1}{3}(x+2)^2-1,[/tex]
is a quadratic equation. In simple words, it's a parabola, whose graph (red curve) is the following:
The axis of symmetry of a parabola is just the line dividing the parabola into its two arms. In the graph, the axis of symmetry is the blue vertical line. It's usually represented algebraically by
[tex]x=\text{ The first component of the vertex}[/tex]
Answer
The axis of symmetry of our quadratic equation is
[tex]x=-2[/tex]
