[tex]a) \\\\\left \{ {{-5=-2a+b} \atop {5=2a+b}} \right. \\-5+5=b+b\\0=b\\\\5=2a /:2\\\frac52=a\\\\y=\frac52x[/tex]
[tex]b) \\\\\left \{ {{0=-5a+b} \atop {3=b}} \right. \\\\0=-5a+3\\5a=3 /:5\\a=\frac35\\\\y=\frac35x+3[/tex]
[tex]c) \\\\\left \{ {{-6=-2a+b} \atop {-6=2a+b}} \right. \\\\-6-6=2b\\-12=2b /:2\\-6=b\\\\-6=-2a-6\\0=-2a /:-2\\a=0\\\\y=-6[/tex]
