a
$\left[\begin{array}{c}x+y+z \\ x+z \\ y+z\end{array}\right]=\left[\begin{array}{l}9 \\ 5 \\ 7\end{array}\right]$
As the two matrices are equal, their corresponding elements are also equal. Comparing the corresponding elements, we get:
$x+y+z=9$ ........... $(1)$
$x+z=5$ ........... $(2)$
$y+z=7$ ........... $(3)$
From $(1)$ and $(2)$, we have :
$y+5=9 \Rightarrow y=4$
From $( 3 )$, we have :
$4+z=7 \Rightarrow z=3$
$\therefore $ $x+z=5 \Rightarrow x=2$
$\therefore $ $x=2$, $y=4,$ and $z=3$