Ответ:

Формула разности кубов   [tex]\bf a^3-b^3=(a-b)(a^2+ab+b^2)[/tex]  .

[tex]\bf y=-\dfrac{1}{5}\ \ ,\\\\\\\underbrace{\bf (5y-2)(25y^2+10y+4)}_{(5y)^3-2^3}+8=(5y)^3-2^3+8=125y^3-8+8=\\\\\\=125y^3=125\cdot \Big(-\dfrac{1}{5^3}\Big)=125\cdot \Big(-\dfrac{1}{125}\Big)=-1[/tex]