解:$(1)A+B=A-B+2B$
$=6x^2y+12xy-2x-9+2(3x^2y-5xy+x+7)$
$=6x^2y+12xy-2x-9+6x^2y-10xy+2x+14$
$=12x^2y+2xy+5$
$(2)$即$A-3B=A-B-2B$
$=6x^2y+12xy-2x-9-2(3x^2y-5xy+x+7)$
$=6x^2y+12xy-2x-9-6x^2y+10xy-2x-14$
$=22xy-4x-23$
$=(22y-4)x-23$的取值与$x$无关
∴$22y-4=0$,得$y=\frac {2}{11}$