How do you simplify (3a)^-3 and write it using only positive exponents?

It would be $\frac{1}{{\left(3 a\right)}^{3}}$
A negative exponent always means the power can be written with a positive exponent by moving that power to the other side of a quotient (a fraction). So, the ${\left(3 a\right)}^{- 3}$ becomes the denominator of a fraction (or the divisor in a quotient if would you rather think of it that way).
Second, since the brackets are placed around the entire $3 a$ monomial, this is the base to which the exponent applies, and not just the $a$ part.