By convention, we consider $[0.1~0.3]$ and $\left[\begin{array}{c}0.1 \\ 0.3 \end{array} \right]$ to be the same.
The matrix $\left[\begin{array}{cc} 1 & 2 \\ 0 & 1 \\ -2 & 1 \end{array}\right]$ has dimensions $2 \times 3$.
The $2,1$ entry of $\left[\begin{array}{cc} 1 & 2 \\ 0 & 1 \\ -2 & 1 \end{array}\right]$ is $0$.
The matrix $\left[\begin{array}{ccc} 1 & 1 & 1 \\ 1 & 1 & 1 \\ 1 & 1 & 1 \end{array}\right]$ is an identity matrix.
Suppose we know that $\left[\begin{array}{c} a \\ b \\ c \end{array}\right] =e_2$, the second unit vector (or standard basis vector). Then we can conclude $a=0$.