3b
Hệ pt⇔$\left \{ {{4(x-2) +6(1+y)=-4} \atop {9(x-2)-6(1+y)=-9}} \right.$ ⇔$\left \{ {{2(x-2)+3(1+y)=-2} \atop {13(x-2)=-13}} \right.$
⇔$\left \{ {{2(x-2)+3(1+y)=-2} \atop {x-2=-1}} \right.$ ⇔$\left \{ {{1+y=0} \atop {x=1}} \right.$
⇔$\left \{ {{y=-1} \atop {x=1}} \right.$
4.
Thay x=3; y=-1 vào hệ pt ta được:
$\left \{ {{6-a=b+4} \atop {3a-b=8+9a}} \right.$⇔$\left \{ {{a+b=2} \atop {6a+b=-8}} \right.$
⇔$\left \{ {{a+b=2} \atop {5a=-10}} \right.$ ⇔$\left \{ {{a=-2} \atop {b=4}} \right.$
