Answer:
x= -10 (you need to enter as well the variable "x" and the equal sign since they want the complete equation of the vertical line of symmetry)
Explanation:
Notice that this equation corresponds to a parabola given in vertex form, with vertex at x = -10, and y = -6.
Such parabola would have an axis of symmetry as a vertical line going through the vertex. Such vertical line should therefore be of the form: x = -10