transformed equation

Hello , please guide me .
How can I transformed the equation $x^{5}-x^{4}-x^{3}-x^{2}-x-1 =0$ to $y^{5}+2y^{2}+47y+122$ ?
I studied a lecture that the writer had written :<< by using $y=x^{2}-3x$ we can transformed $x^{5}-x^{4}-x^{3}-x^{2}-x-1 =0$ to $y^{5}+2y^{2}+47y+122$ But how ? if in $y=x^2-3x$ we obtain $x$ by $y$ we will have : $x=3/2+\sqrt{y+9/4}$ and if we substitute $x=3/2+\sqrt{y+9/4}$ in $x^{5}-x^{4}-x^{3}-x^{2}-x-1 =0$ we don't have $y^5+2y^2+47y+122$ . please explain it.
Thank you very much