[tex]a)\\\\\left \{ {{0=-3a+b} \atop {4=a+b /*3}} \right. \\\left \{ {{0=-3a+b} \atop {12=3a+3b}} \right. \\0+12=b+3b\\12=4b /:4\\3=b\\\\4=a+3 /-3\\1=a\\\\\underline{y=x+3}[/tex]
[tex]b)\\\\\left \{ {{7=-2a+b /*3} \atop {7=6a+b}} \right. \\+\left \{ {{21=-6a+3b \atop {7=6a+b}} \right. \\21+7=3b+b\\28=4b /:4\\b=7\\\\7=-2a+7 /-7\\0=-2a /:(-2)\\a=0\\\\\underline{y=7}[/tex]
