Answer:
[tex]\huge\boxed{-0.001a^{n+1}}[/tex]
Step-by-step explanation:
[tex]-0.001a^{3n+3}=-0.001a^{3(n+1)}=(-0.001a^{n+1})^3\\\\\text{Used}\\\\a(b+c)=ab+ac\qquad\text{distributive property}\\\\(a^n)^m=a^{n\cdot m}[/tex]
