Following the generalized form that you wrote, you know that a negative exponent sends its base to the denominator (or numerator if it's in the denominator already). Or to put it formally as you wrote it, for any nonzero real number a and natural number n, the negative rule of exponents states that [tex]a^{-n} = \frac{1}{a^n} [/tex].
So, [tex]6^{-2}= \frac{1}{6^{2}} = \frac{1}{36} [/tex].
