[tex]\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\\\\y=x+3[/tex]
