x4+x3+2x2+x+1=0
y=x2⟹x=√y∴ replace x with √y in the given question
⟹(√y)4+(√y)3+2(√y)2+√y+1=0
⟹y2+y√y+2y+√y+1=0
⟹√y(y+1)=−(y2+2y+1)
⟹y(y+1)2=(y2+2y+1)2[taking squares on both sides to remove the radical sign]
⟹y(y2+2y+1)=(y4+4y3+6y2+4y+1)
⟹y3+2y2+y=y4+4y3+6y2+4y+1
⟹y4+3y3+4y2+3y+1=0