Chapter 2 - Equations, Inequalities, and Problem Solving - 2.3 Formulas - 2.3 Exercise Set - Page 102: 75

$a=\dfrac{c}{3+b+d}$

Using the properties of equality, in terms of $ a $, the given expression, $ 3a=c-a(b+d) $, is equivalent to \begin{array}{l} 3a=c-ab-ad \\\\ 3a+ab+ad=c \\\\ a(3+b+d)=c \\\\ a=\dfrac{c}{3+b+d} .\end{array}