So this is what we got

-(2/3a)^4

So imagine the 2/3 and a have an imaginary ^1 so it is really

-( 2/3^1 a^1 )^4

so you take the exponet 4 and times it by the other exponets on 2/3 and a

Your final anwser is

-16/81a^4 i believe

someone please correct me if im wrong