Помогите решить, и объясните как решать срочно!

a)[latex] left { {{x+y=5} atop {3x+y=7}} ight. \ left { {{y=5-x} atop {3x+5-x=7}} ight. \ left { {{y=5-x} atop {2x=2}} ight. \ left { {{x=1} atop {y=4}} ight.[/latex]Проверка:[latex] left { {{1+4=5} atop {3*1+4=7}} ight. [/latex]б)[latex]left { {{x-y=0} atop {x-3y=6}} ight. \ left { {{y=x} atop {-2x=6}} ight. \ left { {{x=-3} atop {y=-3}} ight.[/latex]Проверка:[latex] left { {{-3-(-3)=0} atop {-3-3*(-3)=6}} ight. [/latex]в)[latex] left { {{y-x=-3} atop {2x+y=9}} ight. \ left { {{y=x-3} atop {2x+x-3=9}} ight. \ left { {{y=x-3} atop {3x=12}} ight. \ left { {{x=4} atop {y=1}} ight. [/latex]Проверка:[latex] left { {{1-4=-3} atop {2*4+1=9}} ight. [/latex]