Chapter 1 - Introduction to Algebraic Expressions - 1.8 Exponential Notation and Order of Operations - 1.8 Exercise Set - Page 67: 81

$-17$

Substituting $ a $ with $ 5 $ in the given expression, $ \dfrac{3a-4a^2}{a^2-20} $, then, \begin{array}{l}\require{cancel} \dfrac{3(5)-4(5)^2}{(5)^2-20} \\\\= \dfrac{15-4(25)}{25-20} \\\\= \dfrac{15-100}{5} \\\\= \dfrac{-85}{5} \\\\= -17 .\end{array}