I am taking a linear algebra course at my university. We just have the first day of class but I am reading by myself, so I decided to solve some problems. I do not understand how to solve this problem:
"The two systems:
$a)2x + y = 3\tag{1}$ $4x + 3y= 5 \tag{2}$
$b)$ $2x + y = -1\tag{3}$ $4x + 3y= 1 \tag{4}$
have the same coefficient matrix but different right-hand sides. Solve both systems simultaneously by eliminating the $(2,1)$ entry of the augmented matrix
\Bigg(\begin{array}{cc|cc} 2 & 1 & 3 &-1\\ 4 & 4 & 5 &-1\\ \end{array}\Bigg) $
and then performing back substitutions for each columns corresponding to the right hand sides."
What does the(2,1)$ entry mean? Can someone explain me how to solve this? I know that the problem must be easy, but I haven't seen an example yet.