Answer
$x^8+24x^7+252x^6$
Work Step by Step
Apply Binomial Theorem or Binomial expansion to find the first three terms of $(x+3)^8$.
$(x+y)^n=\displaystyle \binom{n}{0}x^ny^0+\displaystyle \binom{n}{1}x^{n-1}y^1+........+\displaystyle \binom{n}{n}x^0y^n$
Now, $(x+3)^8=\displaystyle \binom{8}{0}x^83^0+\displaystyle \binom{8}{1}8^73^1+\displaystyle \binom{8}{2}x^63^2$
$=(1)x^8(1)+8x^7(3)+28x^6(9)$
Thus, $(x+3)^8=x^8+24x^7+252x^6$
