y(y-4)^3-21(y-4)^2=0
(y-4)^2=y^2-8y+16
(y-4)^3=(y^2-8y+16)(y-4)=(y^3-8y^2+16y-4y^2+32y-64)
y(y^3-12y^2+48y-64)-21(y^2-8y+16)=0
(y^4-12y^3+48y^2-64y)-21y^2+168-336=0
y^4-12y^3+48y^2-64y-21y^2+168y-336=0
y^4-12y^3+48y^2-104y-21y^2-336=0
y^4-12y^3+27y^2-104y-336=0