[tex]a) \\\\\left \{ {{3x-5y=3,5/*4} \atop {-4x+2y=-7/*3}} \right. \\+\left \{ {{12x-20y=14} \atop {-12x+6y=-21}} \right. \\-20y+6y=14-21\\-14y=-7 /:(-14)\\y=\frac7{14}=\frac12\\-4x+2*\frac12=-7\\-4x+1=-7 /-1\\-4x=-8 /:(-4)\\x=2[/tex]
[tex]b) \\\left \{ {{2x+3y=15/*(-2)} \atop {2(2x+3y)=30}} \right. \\+\left \{ {{-4x-6y=-30 \atop {4x+6y=30}} \right. \\0=0[/tex]
[tex]\left \{ {{2x+3y=15} \atop {2(3x+3y)=30}} \right. \\2x+3y=15 /-2x\\3y=15-2x /:3\\y=\frac{15-2x}3=\frac{3(5-\frac23x)}3=5-\frac23x[/tex]