[tex]\displaystyle\bf\\1)\\\\\left \{ {{6x+11y=107} \atop {5x-2y=11}} \right. \\\\\\\left \{ {{6x+11y=107} \atop {2y=5x-11}} \right. \\\\\\\left \{ {{6x+11\cdot(2,5x-5,5)=107} \atop {y=2,5x-5,5}} \right. \\\\\\\left \{ {{6x+27,5x-60,5=107} \atop {y=2,5x-5,5}} \right.\\\\\\\left \{ {{33,5x=167,5} \atop {y=2,5x-5,5}} \right. \\\\\\\left \{ {{x=5} \atop {y=2,5\cdot 5-5,5}} \right. \\\\\\\left \{ {{x=5} \atop {y=7}} \right. \\\\\\Otvet: \ (5 \ ; \ 7)[/tex]

[tex]\displaystyle\bf\\\left \{ {{5x-6y=9} \ |\cdot(-3) \atop {15x-18y=26}} \right. \\\\\\+\left \{ {{-15x+18y=-27} \atop {15x-18y=26}} \right. \\------------\\0\cdot x+0\cdot y=-1[/tex]

Ответ : решений нет

[tex]\displaystyle\bf\\3)\\\\\left \{ {{4x-ay=3} \atop {20x+10y=15}} \right.[/tex]

Система имеет бесконечно много решений , когда выполняется условие :

[tex]\displaystyle\bf\\\frac{4}{20} =\frac{-a}{10} =\frac{3}{15} \\\\\\\frac{1}{5} =\frac{-a}{10} =\frac{1}{5} \\\\\\\frac{1}{5} =\frac{-a}{10}\\\\\\-5a=10\\\\\\\boxed{a=-2}[/tex]