Chapter 1 - Introduction to Algebraic Expressions - 1.8 Exponential Notation and Order of Operations - 1.8 Exercise Set - Page 66: 76

$\dfrac{1}{8}$

Substituting $ a $ with $ 2 $, then the given expression, $ 6a\div12a^3 $, evaluates to \begin{array}{l}\require{cancel} 6(2)\div12(2)^3 \\\\= 12\div12(8) \\\\= \dfrac{12}{12(8)} \\\\= \dfrac{\cancel{12}}{\cancel{12}(8)} \\\\= \dfrac{1}{8} .\end{array}