In the given equation:
[tex]\text{ y = -(x + 3)}^2\text{ + 5}[/tex]The equation already in vertex form,
[tex]\text{ y = a(}x-h)^2\text{ + k}[/tex]The axis of symmetry is h.
From the equation,
[tex]\text{ y = -(x + 3)}^2\text{ + 5}[/tex]The axis of symmetry is therefore,
[tex]\text{ x + 3 = 0 }\rightarrow\text{ x }=\text{ -3}[/tex]