TeX source:
\sum_{k=0}^{3} \binom{3}{k} (3x)^{n-k}(-2)^k = \binom{3}{0}(3x)^3(-2)^0 + \binom{3}{1}(3x)^2(-2)^1 + \binom{3}{2}(3x)^1(-2)^2 + \binom{3}{3}(3x)^0(-2)^3 = 27x^3 - 54x^2 + 36x - 8