for #7 multiply the 2nd equation by -1 and you will have
[tex]x + 4y = -3[/tex]
[tex] - x - 7y = 12[/tex]
add those two together to get y= -3 and plug that into any equation, so take
[tex]x + 4( - 3) = - 3[/tex]
[tex]x - 12 = - 3[/tex]
[tex]x = 9[/tex]
[tex]x = 9 \\ y = - 3[/tex]
