Способ постановки решить систему уравнений: 1)7x–6y=20 2)5x+9y=–1 3x–y=7 x+2y=4 Очень...

Решите задачу:

$a)\left \{ {{7x-6y=20} \atop {3x-y=7}} \right.;\left \{ {{7x-6y=20} \atop {-18x+6y=-42}} \right.; \frac{\left \{ {{7x-6y=20} \atop {-18x+6y=-42}} \right.}{7x-18x-6y+6y=20-42}}; \\ \\ \left \{ {{14-6y=20} \atop {x=2}} \right.; \left \{ {{y=-1} \atop {x=2}} \right. \\ \\ \\ b) \left \{ {{5x+9y=-1} \atop {x+2y=4}} \right.;\left \{ {{5x+9y=-1} \atop {-5x-10y=-20}} \right.; \frac{\left \{ {{5x+9y=-1} \atop {-5x-10y=-20}} \right.}{5x-5x+9y-10y=-1-20}}; \\ \\ \left \{ {{y=-21} \atop {x-42=4}} \right.; \left \{ {{y=-21} \atop {x=46}} \right.$