How? I don't fully understand this

Answer: A
Step-by-step explanation:
(9*10^9)*(-2*10^-3)=
9*(-2)*10^9*10*-3=
-18*10^(9+(-3))=
-18*10^(9-3)=-18*10^6 ==> A
[tex](9\cdot 10^9)\cdot (-2\cdot 10^{-3})\implies (9)(-2)\qquad (10^9\cdot 10^{-3}) \\\\\\ (-18)\qquad (10^{9-3})\implies -18\cdot 10^6[/tex]