Answer :
Since you've multiplied the firs equation by 4 you have a 12x. So you must found a number that multiplied by the 4x in the second equation will result in -12x. Concerning that the negative signal turns the number to negative, you come to the answer -3 (Minus three)
[tex] \left \{ {{3x-2y=4\ \ \ |*4} \atop {4x+5y=28}} \right. \\\\ \left \{ {{12x-8y=16} \atop {4x+5y=28}} \right.\\\\To\ eliminate\ by\ adding\ multiply\ second\ equation\ by\ -3\ to\\ obtain\ -12x\\\\
\left \{ {{12x-8y=16} \atop {-12x-15y=-84}} \right.\\+----\\Addition:\\\\
-8y-15y=16-84\\
-33y=-68\\
y=\frac{68}{33}\\
4x=28-5y\\
x=\frac{28-5y}{4}=\frac{28-5*\frac{68}{33}}{4}=\frac{1}{4}
[/tex]