Answer
$\begin{bmatrix}21 & -1\\-1 &13\end{bmatrix}$
Work Step by Step
We have $BB^{T}=\begin{bmatrix}4 & -2 & 1 \\0 & 2 & 3\end{bmatrix} \cdot \begin{bmatrix}4 & 0\\-2 & 2 \\1 & 3\end{bmatrix}$
$=\begin{bmatrix}16+4+1 & 0-4+3 \\0-4+3 & 0+4+9\end{bmatrix}$
After simplification, we obtain:
$=\begin{bmatrix}21 & -1\\-1 &13\end{bmatrix}$
