stumper

I have tried this sum 3 times and got 3 different answers, please help me with this one:

\(\mathbf{\large{ \dfrac{ 54^n-18^{n-1}\cdot 3^{n+1} } { {(3^{n-1})}^2\cdot 6^n } }}\)

\(\begin{array}{|rcll|} \hline &&\mathbf{ \dfrac{ 54^n-18^{n-1}\cdot 3^{n+1} } { {(3^{n-1})}^2\cdot 6^n } } \\\\ &=& \dfrac{ 54^n-18^n\cdot 18^{-1}\cdot 3^n\cdot 3^1 } { { 3^{(n-1)\cdot 2} } \cdot 6^n } \\\\ &=& \dfrac{ 54^n-18^n\cdot 3^n\cdot \frac{3}{18}} { {3^{2n-2}}\cdot 6^n } \\\\ &=& \dfrac{ 54^n-(18\cdot 3)^n\cdot \frac{1}{6} } { {3^{2n}\cdot 3^{-2} }\cdot 6^n } \\\\ &=& \dfrac{ 54^n-54^n\cdot \frac{1}{6} } { \left(3^2\right)^n\cdot \frac{1}{9} \cdot 6^n } \\\\ &=& \dfrac{ 54^n \cdot \left(1- \frac{1}{6}\right) } { 9^n\cdot 6^n \cdot \frac{1}{9} } \\\\ &=& \dfrac{ 54^n \cdot \frac{5}{6} } { \left(6\cdot 9\right)^n \cdot \frac{1}{9} } \\\\ &=& \dfrac{ 54^n \cdot \frac{5}{6} } { 54^n \cdot \frac{1}{9} } \\\\ &=& \dfrac{ \frac{5}{6} } { \frac{1}{9} } \\\\ &=& \dfrac{5}{6} \cdot \dfrac{9}{1} \\\\ &=& \dfrac{5\cdot 3}{2} \\\\ &\mathbf{=}& \mathbf{\dfrac{15}{2}} \\ \hline \end{array}\)