（-8a^3b^2）^2(-4a^2b^3)^3÷(-16a^7b^9)
=64a^6b^4*(-64a^6b^9)÷(-16a^7b^9)
=-64*64a^12b^13÷(-16a^7b^9)
=64*4a^5b^4
=256a^5b^4