Verified answer

[tex]1. \quad12ab^4 = 3ab^4\cdot 4 = (-7)\cdot 4 = -28.\\2. \quad3a^2b^8 = 3ab^4\cdot ab^4 = (-7)\cdot \frac{3ab^4}{3} = (-7)\cdot \frac{-7}{3} = \frac{49}{3}.[/tex]

Ответ: -28; [tex]\frac{49}{3}.[/tex]