Verified answer

[tex]\displaystyle\bf\\\left \{ {{2x + y = 3 \: \: | \times 4} \atop {ax + 4y = 12 }} \right. \\ \displaystyle\bf\\\left \{ {{8x + 4y = 12} \atop {ax + 4y = 12 }} \right. \\ \\ a = 8[/tex]