Question: factor $x^2 + 6x + 9$
1. **State the problem:** Factor the quadratic expression $x^2 + 6x + 9$.
2. **Recall the formula:** A quadratic expression $ax^2 + bx + c$ can be factored as $(x + m)(x + n)$ where $m$ and $n$ satisfy $m + n = b$ and $mn = c$.
3. **Identify coefficients:** Here, $a = 1$, $b = 6$, and $c = 9$.
4. **Find two numbers:** We need two numbers $m$ and $n$ such that:
$$m + n = 6$$
$$mn = 9$$
5. **Check possible pairs:** The pairs that multiply to $9$ are $(1, 9)$ and $(3, 3)$. Only $(3, 3)$ adds up to $6$.
6. **Write the factorization:** Therefore,
$$x^2 + 6x + 9 = (x + 3)(x + 3) = (x + 3)^2$$
7. **Final answer:** The factored form is $\boxed{(x + 3)^2}$.