(ab)¯³

a-³ x b-³ =
1/a³ x 1/b³=

1/a^2b^2

why is the exponent changed to 2 and does not remain 3? Thanks

Because x often is used an unknown, * is used online to indicate multiplication.

From your data, I don't know how you get (1/a)^2 * b^2 either.