(1)∵A﹣2B=﹣7x 2+10x+12,B=4x 2﹣5x﹣6,
∴A=﹣7x 2+10x+12+2(4x 2﹣5x﹣6)=x 2,
∴A+2B=x 2+2(4x 2﹣5x﹣6)=9x 2﹣10x﹣12;
(2)当x=3时,A+2B=9×3 2﹣10×3﹣12=39;
(1)∵A﹣2B=﹣7x 2+10x+12,B=4x 2﹣5x﹣6,
∴A=﹣7x 2+10x+12+2(4x 2﹣5x﹣6)=x 2,
∴A+2B=x 2+2(4x 2﹣5x﹣6)=9x 2﹣10x﹣12;
(2)当x=3时,A+2B=9×3 2﹣10×3﹣12=39;