Question

X + y = 12
2x - y = 3

Answer 1

$\left \{ {{x+y=12} \atop {2x-y=3}} \right.$

$(x+y)+(2x-y)=12+3$

$x+y+2x-y=15$

$3x=15$

$x=\frac{15}{3}$

$x=5$

$x+y=12$

$5+y=12$

$y=12-5$

$y=7$