Answer
$y^{63}-21y^{60}+210x^{57}$
Work Step by Step
Apply Binomial Theorem or Binomial expansion to find the first three terms of $(y^3-1)^{21}$.
$(x+y)^n=\displaystyle \binom{n}{0}x^ny^0+\displaystyle \binom{n}{1}x^{n-1}y^1+........+\displaystyle \binom{n}{n}x^0y^n$
Now, $(y^3-1)^{21}=\displaystyle \binom{21}{0}(y^3)^{21}(-1)^0+\displaystyle \binom{21}{1}(y^3)^{20}(-1)^1+\displaystyle \binom{21}{2}(y^3)^{19}(-1)^2$
Thus, $(y^3-1)^{21}=y^{63}-21y^{60}+210x^{57}$
