Use the summation formulas to rewrite the expression ${\displaystyle \underset{k=1}{\overset{n}{}}}\frac{18k(k-1)}{n^6}$ without the summation notation.

$\frac{6}{n^4}-\frac{3}{n^5}$

$\frac{3}{n^4}-\frac{1}{n^5}$

$\frac{6}{n^3}+\frac{2}{n^6}$

$\frac{3}{n^3}-\frac{3}{n^5}$

$\frac{6}{n^3}-\frac{6}{n^5}$