a) $(y-2)^3-(y-3)(y^2+3y+9)+6(y+1)^2=49$
$⇔ y^3-6y^2+12y-8-y^3+27+6(y^2+2y+1)=49$
$⇔ -6y^2+12y+25+6y^2+12y=49$
$⇔ 24y+25=49$
$⇔ 24y=24$
$⇔ y=1$
b) $(y+3)^3-(y+1)^3=56$
$⇔ y^3+9y^2+27y+27-y^3-3y^2-3y-1=56$
$⇔ 6y^2+24y+26=56$
$⇔ 6y^2+24y-30=0$
$⇔ 6y^2-6y+30y-30=0$
$⇔ (6y+30)(y-1)=0$
$⇔ y=-5$ hoặc $y=1$