Multiply $\frac{\left(y^{2}+y+1\right)\left(y-1\right)}{\left(y+1\right)\left(y+1\right)}$ times $\frac{y+1}{y^{2}+y+1}$ by multiplying numerator times numerator and denominator times denominator.
Multiply $\frac{\left(y^{2}+y+1\right)\left(y-1\right)}{\left(y+1\right)\left(y+1\right)}$ times $\frac{y+1}{y^{2}+y+1}$ by multiplying numerator times numerator and denominator times denominator.