Answer
$x^3+9x^2y+27xy^2+27y^3$
Work Step by Step
Binomial Theorem or Binomial expansion can be defined as:
$(x+y)^n=\displaystyle \binom{n}{0}x^ny^0+\displaystyle \binom{n}{1}x^{n-1}y^1+........+\displaystyle \binom{n}{n}x^0y^n$
Need to apply the formula to get the Binomial Expansion.
we have
$(x+3y)^3=\displaystyle \binom{3}{0}(x)^3(3y)^0+\displaystyle \binom{3}{1}(x^{2})(3y)^1
+\displaystyle \binom{3}{2}(x)^1(3y)^2+\displaystyle \binom{3}{3}(x)^0(3y)^3$
$=x^3(1)+(3)(x^2)(3y)+3(x)(9y^2)+27(y^3)(1)$ $\bf{(Simplify)}$
$=x^3+9x^2y+27xy^2+27y^3$
