Benjamin is taking Algebra 1 and is stuck on the problem shown below. Examine his work so far and help him by showing and explaining the remaining steps.

Original problem: Simplify $(3a^{−2}b)^3.$ 
He knows that $(3a^{−2}b)^3=(3a^{−2}b)(3a^{−2}b)(3a^{−2}b)$. Now what?