2. (5 points) If A and B are 4 x 4 matrices that are row-equivalent, and the only solution \text { to } B \vec{x}=\left[\begin{array}{l} 0 \\ 0 \\ 0

\end{array}\right] \text { is } \vec{x}=\overrightarrow{0}, \text { prove that } A \text { is invertible. }