An exponent with a negative power is equal to the reciprocal of the same number; \(a^{-n} = \frac{1}{a^{n}}\)

\(3^{-3}+3^{-2}+3^{-1}\)
\(= \frac{1}{3^3} + \frac{1}{3^2} + \frac{1}{3^1}\)
\(= \frac{1}{27} + \frac{1}{9} + \frac{1}{3}\)
\(= \frac{1}{27} + (\frac{1}{9}*\frac{3}{3}) + (\frac{1}{3}*\frac{9}{9})\)
\(= \frac{1}{27} + \frac{3}{27} + \frac{9}{27}\)
\(= \frac{13}{27}\)
\(= \frac{13}{3^{3}}\)
\(= 13(3^{-3})\)