axis of symmetry

y=-(x+6)^2-3

This is a parabola that turns downward whose vertex is (-6, -3)

The axis of symmetry is given by   x = the x coordinate of the vertex

So.......the axis of symmetry  is     ....     x = -6

Here's a graph  of the parabola and the axis  ........https://www.desmos.com/calculator/mwnmgvdtg8