The rank is the number of nonzero rows of a (reduced) row echelon form matrix of the given matrix.

We apply elementary row operations as follows.
\begin{align*}
&\begin{bmatrix}
a & 1 & 2 \\
1 &1 &1 \\
-1 & 1 & 1-a
\end{bmatrix}
\xrightarrow{R_1 \leftrightarrow R_2}
\begin{bmatrix}
1 &1 &1 \\
a & 1 & 2 \\
-1 & 1 & 1-a
\end{bmatrix}
\xrightarrow[R_3+R_1]{R_2-aR_1}
\begin{bmatrix}
1 &1 &1 \\
0 & 1-a & 2-a \\
0 & 2 & 2-a
\end{bmatrix}\\[8pt]
&\xrightarrow{R_2\leftrightarrow R_3}
\begin{bmatrix}
1 &1 &1 \\
0 & 2 & 2-a\\
0 & 1-a & 2-a
\end{bmatrix}
\xrightarrow{R_3-\frac{1-a}{2}R_2}
\begin{bmatrix}
1 &1 &1 \\
0 & 2 & 2-a\\
0 & 0 & (2-a) (a+1)/2
\end{bmatrix}.
\end{align*}

The last matrix is in row echelon form.
Therefore, if $a \neq -1, 2$, then $(3, 3)$-entry of the last matrix is not zero. From this we see that the rank is $3$ when $a \neq -1, 2$.

On the other hand, when $a=-1$ or $a=2$ the third row is a zero row, hence the rank is $2$.

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\[\mathbf{x}_1=\begin{bmatrix}
1 \\
1 \\
1
\end{bmatrix}, \mathbf{x}_2=\begin{bmatrix}
0 \\
1 \\
1
\end{bmatrix}, \mathbf{x}_3=\begin{bmatrix}
0 \\
0 \\
1
\end{bmatrix} \]
into the […]

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For which choice(s) of the constant $k$ is the following matrix invertible?
\[A=\begin{bmatrix}
1 & 1 & 1 \\
1 &2 &k \\
1 & 4 & k^2
\end{bmatrix}.\]
(Johns Hopkins University, Linear Algebra Exam)
Hint.
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\[\left(\, f\left(\, \begin{bmatrix}
\alpha \\
\beta
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\[\begin{bmatrix}
1 \\
2 \\
0
\end{bmatrix}, \begin{bmatrix}
2 \\
1 \\
0
\end{bmatrix}, \text{ and } \begin{bmatrix}
1 \\
-1 \\
0
[…]

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(a) Let $A=\begin{bmatrix}
1 & 3 & 0 & 0 \\
1 &3 & 1 & 2 \\
1 & 3 & 1 & 2
\end{bmatrix}$.
Find a basis for the range $\calR(A)$ of $A$ that consists of columns of $A$.
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(a) […]

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For each of the following $3\times 3$ matrices $A$, determine whether $A$ is invertible and find the inverse $A^{-1}$ if exists by computing the augmented matrix $[A|I]$, where $I$ is the $3\times 3$ identity matrix.
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1 & 3 & -2 \\
2 &3 &0 \\
[…]