Solution
= Solution
First solve $L\mathbf y=\mathbf b$ by forward substitution:
$$
\mathbf y=(-3,-6,0,-2)^T.
$$
Then solve $U\mathbf x=\mathbf y$ by backward substitution, obtaining
$$
\boxed{\mathbf x=(1,-1,1,-1)^T}.
$$
Direct substitution verifies $A\mathbf x=\mathbf b$.