Answer
$(2/a,3/a)$
Work Step by Step
We will write the system
$\left\{\begin{array}{r}{2x+y=\dfrac{7}{a}}\\{ax+ay=5}\end{array}\right.$
in matrix form as: $AX=B$
where, $X=\left[\begin{array}{l}x\\y \end{array}\right]$
We have:
$A=\left[\begin{array}{ll}{2}&{1}\\{a}&{a}\end{array}\right]$
and its inverse is:
$A^{-1}=\left[\begin{array}{rr}{1}&{-1/a}\\{-1}&{2/a}\end{array}\right]$
Thus, the solution of the given matrix can be expressed as:
$X=A^{-1}B=\left[\begin{array}{rr}{1}&{-1/a}\\{-1}&{2/a}\end{array}\right]\left[\begin{array}{l}
\dfrac{7}{a}\\5\end{array}\right]$
$\left[\begin{array}{l}
x\\y \end{array}\right]=\left[\begin{array}{l} \dfrac{7}{a}-\dfrac{5}{a}\\
\dfrac{-7}{a}+\dfrac{10}{a} \end{array}\right]=\left[\begin{array}{l}
2/a\\3/a\end{array}\right]$
So, our solution is: $(x,y)=(2/a,3/a)$