[(-3ab)^2a^3-2a^2(3ab^2))^3*1/2b]÷(-3ab^2))^2
=(9a^2b^2*a^3-2a^2*27a^3b^6*b/2)/(9a^2b^4)
=(9a^5b^2-27a^5b^7)/(9a^2b^4)
=(a^3-3a^3b^5)/(b^2).