The Span and Uniqueness of Solutions in a Parametric Matrix

Consider the matrix:

$$A = \begin{pmatrix} 2 & -1 & 4 \\ a & 0 & b \\ 1 & 2 & 7 \end{pmatrix} $$

a) Decide if the rows in A would span $\mathbb{R}^{3} $ when a = 1 and b = 3

b) Let a = 2 and b = 0. Explain how Ax = b has exactly one solution for every $b \in \mathbb{R} ^{3}$. Give a simple, short formula for the solution.