Answer
$x^8+24x^7+252x^6$
Work Step by Step
Calculate the first three terms of $(x+3)^8$ by using Binomial Theorem or Binomial expansion.
This implies $(x+3)^8=\displaystyle \binom{8}{0}(x)^83^0+\displaystyle \binom{8}{1}(x)^73^1+\displaystyle \binom{8}{2}(x)^63^2$
or, $=x^8+8x^7(3)+(28)x^6(9)$
or, $=x^8+24x^7+252x^6$
