Find the echelon form $U$, the free variables, and the special solutions:
$$
A=\left[\begin{array}{llll}
0 & 1 & 0 & 3 \\
0 & 2 & 0 & 6
\end{array}\right], \quad b=\left[\begin{array}{l}
b_{1} \\
b_{2}
\end{array}\right]
$$
$A x=$ ______ b is consistent (has a solution) when $b$ satisfies $b_{2}=$ Find the complete solution in the same form as equation (4).