What we have here is a parabola written in the y=a(x-h)^2+k
(h,k) is the vertex of the parabola. So, this parabola has a vertex at (7,-2). Since this is a parabola that is concave up, the line of symmetry will be a vertical line passing through point (7, -2). So, it must be x=7. This is the axis of symmetry.
i hope this helps out.
D. Y. T.