I can find the first $x$, but not the other. I know that the other $x$ is equal to $-4$.
$$\begin{align*} 1 &= (x^2+2x+1)-8\\ 9&=(x^2+2x+1)\\ 9&= (x_1+1)^2\\ 3&= x_1+1\\ 2&= x_1 \end{align*}$$
How do I deduce what the other $x$ is by using the method "completing the square"?
Caution:
$$(x+1)^2=9\implies x+1=\color{red}{\pm 3}\implies\begin{cases}x_1+1=-3\implies x_1=-4\\{}\\x_2+1=3\implies x_2=2\end{cases}$$