For any square linear system $\,A\vec x=\vec b\,$ over some field, there exists a unique solution iff $\,\det A\neq 0\,$ , as then we can use the inverse matrix:
$$A\vec x=\vec b\Longleftrightarrow A^{-1}A\vec x=A^{-1}\vec b\Longleftrightarrow A^{-1}\vec b=\vec x $$
As for (a) and your "main question": if $\,\det A=0\,$ one still may have to check whether there are no solutions or infinite solutions (assuming we're working on an infinite field). For example, if the system is homogeneous (over an infinite field) it must have infinite solutions, whereas if the system is non-homogeneous it may have no solutions or several:
$$\begin{cases}x+y=1\\x+y=1\end{cases} \Longleftrightarrow \begin{pmatrix}1&1\\1&1\end{pmatrix}\binom{x}{y}=\binom{1}{1}\longrightarrow\,\,\text{infinite solutions}$$
$$\begin{cases}x+y=1\\x+y=0\end{cases} \Longleftrightarrow \begin{pmatrix}1&1\\1&1\end{pmatrix}\binom{x}{y}=\binom{1}{0}\longrightarrow\,\,\text{no solutions at all}$$
and, of course, in both cases above we have $\,\det A=0\,$
When using Cramer's rule, there will be no variables whatsoever, rather, you know that the coordinates of the solution to $Ax=b$ are given by $$x_i=\frac{\det{(A\mid i)}}{\det A}$$ where $(A|i)$ is the matrix obtained by replacing $A$'s $i-th$ column with the column vector $b$. I give you an example on how to compute $\det A$, and maybe you can compute the remaining determinants yourself. Recall how how the detereminant behaves:
$({\rm i})$ It remains unchanged if we sum a multiple of a row (column) to another row (column)
$(\rm ii)$ It changes sign if we permute two rows (columns)
$(\rm iii)$ Scalars hop off the determinant.
Thus you may partially triagulate your matrix and operate as follows
$$\begin{align}
\det \left( {\begin{matrix}
1 & 1 & { - 2} & 3 \\
0 & { - 2} & 0 & { - 1} \\
0 & 7 & 3 & 0 \\
0 & 9 & 5 & { - 1} \\
\end{matrix} } \right) &= - \det \left( {\begin{matrix}
1 & 1 & { - 2} & 3 \\
0 & 2 & 0 & 1 \\
0 & 7 & 3 & 0 \\
0 & 9 & 5 & { - 1} \\
\end{matrix} } \right) \cr
\text{cofactor} &= - \det \left( {\begin{matrix}
2 & 0 & 1 \\
7 & 3 & 0 \\
9 & 5 & { - 1} \\
\end{matrix} } \right) \cr
R_2-3R_1\to R_2'\;,R_3-9R'_2\to R_3'\;\; &= - \det \left( {\begin{matrix}
0 & { - 6} & 7 \\
1 & 3 & { - 3} \\
0 & { - 22} & {26} \\
\end{matrix} } \right) \cr
\text{ permute rows } &= \det \left( {\begin{matrix}
1 & 3 & { - 3} \\
0 & { - 6} & 7 \\
0 & { - 22} & {26} \\
\end{matrix} } \right) \cr
\text{ cofactor } &= \det \left( {\begin{matrix}
{ - 6} & 7 \\
{ - 22} & {26} \\
\end{matrix} } \right) = - 2 \end{align} $$
You can also triangulate it, and just calculate $\prod a_{ii}$ which is almost what I did above. Here is W|A's computation.
ADD There is a formula that does not involve cofactors, but it involves $4!=24$ terms, namely $$\det A=\sum_{\sigma \in S_4}\operatorname{sgn}\sigma a_{1\sigma(1)}\cdots a_{n\sigma(n)}$$
Best Answer
Two exercises that may give you the answer you need (no work, no gain):