Since $\frac{y^{2}\left(y^{2}+y+1\right)}{y^{2}+y+1}$ and $\frac{\left(y^{2}-1\right)\left(y+1\right)}{y^{2}+y+1}$ have the same denominator, subtract them by subtracting their numerators.
Since $\frac{y^{2}\left(y^{2}+y+1\right)}{y^{2}+y+1}$ and $\frac{\left(y^{2}-1\right)\left(y+1\right)}{y^{2}+y+1}$ have the same denominator, subtract them by subtracting their numerators.