There are two ways to interpret this problem.
First interpret way:
\(\frac{-5}{{6}^{3}}\)
\(-\frac{5}{{6}^{3}}\)
\(-\frac{5}{216}\)
\(-0.0231481481481481...\)
Second interpert way:
\(({\frac{-5}{6}})^{3}\)
\(-({\frac{5}{6}})^{3}\)
\(-\frac{125}{216}\)
\(-0.5787037037037037...\)