Answer :

AyaZ
[tex](3*4)+(6*a)=30[/tex]
[tex]6a=18 \\ a=3[/tex]
OR
[tex](-2*4)+(5*a)=7[/tex]
[tex]5a=15 \\ a=3[/tex]
[tex] \left[\begin{array}{ccc}3&6\\-2&5\end{array}\right] \cdot \left[\begin{array}{ccc}4\\a\end{array}\right] = \left[\begin{array}{ccc}30\\7\end{array}\right] \\ \left[\begin{array}{ccc}3\cdot4+6\cdot a\\-2\cdot4+5\cdot a\end{array}\right] = \left[\begin{array}{ccc}30\\7\end{array}\right] \\ \left[\begin{array}{ccc}12+6a\\-8+5a\end{array}\right] = \left[\begin{array}{ccc}30\\7\end{array}\right] \\\\ 12+6a=30\\ -8+5a=7\\\\ 6a=18\\ 5a=15\\\\ a=3\\ a=3\\\\ \boxed{a=3}[/tex]

I'm not good at this stuff. There may a quicker and better way to solve it.

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