1. **State the problem:** Find the line of symmetry for the quadratic function $f(x) = x^2 + 10x + 9$ given the vertex $(-5, -16)$. 2. **Recall the formula:** The line of symmetry for a parabola given by $f(x) = ax^2 + bx + c$ is the vertical line passing through the vertex's $x$-coordinate. 3. **Apply the rule:** Since the vertex is at $(-5, -16)$, the line of symmetry is the vertical line $x = -5$. 4. **Explain:** This line divides the parabola into two mirror-image halves. Every point on one side has a corresponding point on the other side at the same distance from $x = -5$. **Final answer:** The line of symmetry is $x = -5$.