Answer
$x^{9}-18x^8y+144x^7y^2$
Work Step by Step
Calculate the first three terms of $(x-2y)^{9}$ by using Binomial Theorem or Binomial expansion.
This implies
$(x-2y)^{9}=\displaystyle \binom{9}{0}(x)^{9}(-2y)^0+\displaystyle \binom{9}{1}(x)^{8}(-2y)^1+\displaystyle \binom{9}{2}(x)^{7}(-2y)^2$
or, $=x^{9}+(9)x^8(-2y)+(36)x^7(4y^2)$
or, $=x^{9}-18x^8y+144x^7y^2$
