Which are the effects of the multiplication on the matrix A? I am not understanding what the following question is asking:

b) Which are the effects of the multiplication on the matrix A?

Where:
$$A = \left (\begin{matrix}1 & 1 & 1 \\ 1 & 2 & 3 \\ 2 & -1 & 5\end{matrix} \right)$$
$$I_{12} = \left (\begin{matrix}0 & 1 & 0 \\ 0 & 0 & 0 \\ 0 & 0 & 0\end{matrix} \right)$$
The first question was asking:

a) Compute or state "not defined" $I_{12}$A.

And I answered that the multiplication is defined and the resulted matrix is:
$$I_{12} = \left (\begin{matrix}1 & 1 & 3 \\ 0 & 0 & 0 \\ 0 & 0 & 0\end{matrix} \right)$$
What I am not understanding is what they are asking in point b), could you please clarify me?
 A: Let $I_{ij}$ be a matrix with zeros, except for row $i$, column $j$ where the entry is $1$. E.g. $$ I_{12} = \left (\begin{matrix}0 & 1 & 0 \\ 0 & 0 & 0 \\ 0 & 0 & 0\end{matrix} \right)$$
Then $I_{ij}A$ will be a matrix where the $i$th row will be the $j$th row of $A$, and the rest of the rows will be zeros. For our example we get
$$I_{12}A = \left (\begin{matrix}1 & 2 & 3 \\ 0 & 0 & 0 \\ 0 & 0 & 0\end{matrix} \right). $$
As you can see the resultant matrix has the $1$st row be the $2$nd row of $A$, and the rest zero.
Similarly, the matrix $A I_{ij}$ will have the $j$th column be the $i$th column of $A$, and the rest of the columns will be zero. For your example you will get
$$AI_{12} = \left (\begin{matrix}0 & 1 & 0 \\ 0 & 1 & 0 \\ 0 & 2 & 0\end{matrix} \right). $$
As you can see the resultant matrix has the $2$nd column be the $1$st column of $A$, and the rest are zero.
A: You made a typo or something with your resulting matrix. The real answer is:
$$I_{12}A=\left (\begin{matrix} 1 &2&3\\ 0 & 0 &0\\ 0& 0 & 0\end{matrix}\right )$$
Question (b) is probably refering to the fact that multiplying $A$ by $I_{12}$ gives you a matrix whose first row is the second row of $A$, and whose other rows are all null. 
