Uh, I think this is so because the quadratic graph is symmetrical about the y-axis. And the equation of line of symmetry usually is x is equals to something since the line of symmetry is a vertical line. Hence the equation of line of symmetry in this case is t=0 because the line is at the y axis where the value of x axis is 0
Your axis of symmetry is always going to be related to the x-point of your vertex. In this case, it looks like the vertex of the parabola is (0, 9). The vertex is the highest (or lowest) point of the parabola and is even on both sides. This means that, in order to be symmetrical, you would need to divide this parabola down "0."
