Answer
$y^{63}-21y^{60}+210y^{57}$
Work Step by Step
Calculate the first three terms of $(y^3-1)^{21}$ by using Binomial Theorem or Binomial expansion.
This implies
$(y^3-1)^{21}=\displaystyle \binom{21}{0}(y^3)^{21}(-1)^0+\displaystyle \binom{21}{1}(y^3)^{20}(-1)^1+\displaystyle \binom{21}{2}(y^3)^{19}(-1)^2$
or, $=(1)y^{63}(1)-21y^{60}+210y^{57}(1)$
or, $=y^{63}-21y^{60}+210y^{57}$
