Помогите решить, и объясните как решать срочно!

A)
$\left \{ {{x+y=5} \atop {3x+y=7}} \right. \\ \left \{ {{y=5-x} \atop {3x+5-x=7}} \right. \\ \left \{ {{y=5-x} \atop {2x=2}} \right. \\ \left \{ {{x=1} \atop {y=4}} \right.$
Проверка:
$\left \{ {{1+4=5} \atop {3*1+4=7}} \right.$
б)
$\left \{ {{x-y=0} \atop {x-3y=6}} \right. \\ \left \{ {{y=x} \atop {-2x=6}} \right. \\ \left \{ {{x=-3} \atop {y=-3}} \right.$
Проверка:
$\left \{ {{-3-(-3)=0} \atop {-3-3*(-3)=6}} \right.$
в)
$\left \{ {{y-x=-3} \atop {2x+y=9}} \right. \\ \left \{ {{y=x-3} \atop {2x+x-3=9}} \right. \\ \left \{ {{y=x-3} \atop {3x=12}} \right. \\ \left \{ {{x=4} \atop {y=1}} \right.$
Проверка:
$\left \{ {{1-4=-3} \atop {2*4+1=9}} \right.$