Solution (source code)

= Solution

As $x_0=0$, solve $(D-L)x_1=b$. Its first row gives $4(x_1)_1=1$; its second gives $(x_1)_1+2(x_1)_2=-1$. Therefore
$$
\boxed{x_1=\begin{pmatrix}1/4\\-5/8\end{pmatrix}.}
$$
The second component uses the new first component $1/4$, distinguishing <Gauss-Seidel iteration> from <Jacobi iteration>.