đáp án m=2
b)$\left \{ {{mx+y=5} \atop {2x-y=-2}} \right.$ $\left \{ {{mx+y=5} \atop {2x-y=-2}} \right.$
⇔$\left \{ {{2x+mx=3} \atop {2x-y=-2}} \right.$
⇔$\left \{ {{x(2+m)=3} \atop {2x-y=-2}} \right.$
⇔$\left \{ {{x=\frac{3}{2+m} } \atop {\frac{6}{2+m}-y=-2 }} \right.$
⇔$\left \{ {{x=\frac{3}{2+m} } \atop {y=2+ \frac{6}{2+m}}} \right.$
để 2x+3y=12 thì
--->$\frac{6}{2+m}$ +$\frac{12+6m+18}{2+m}$ =12
⇔36+6m=24+12m
⇔-6m=-12
⇔m=2