7. Let \( \mathbf{A} \) be an \( m\times n \) matrix. The linear system \( \mathbf{A}\mathbf{x} = \mathbf{b} \) has at most one solution for every \( \mathbf{b}\in \mathbb{R}^m \) if and only if the column vectors of \( \mathbf{A} \) are linearly independent.