Answer :
[tex]-a^3b (3a^2 b^5 - ab^4 - 7a ^2 b)=\\
-3a^5b^6+a^4b^5+7a^5b^2[/tex]
[tex]-a^3b (3a^2 b^5 - ab^4 - 7a ^2 b) \\ \\ =-3a^5b^6-(-a^4b^5)-(-7a^5b^2) \\ \\ =-3a^5b^6+a^4b^5+7a^5b^2[/tex]