Equivalence theorem for nonsingular matrices: the columns of A span R^n (or C^n).
Theorem: An $n \times n$ matrix $A$ is nonsingular iff the columns of $A$ span $\mathbb{R}^n$ (or $\mathbb{C}^n$). The column space equals the entire space, so $Ax=b$ is solvable for every $b$.