Chapter 2 - Equations, Inequalities, and Problem Solving - 2.2 Using the Principles Together - 2.2 Exercise Set - Page 94: 63

$a=2$

Using the properties of equality, the solution to the given equation, $ 7(2a-1)=21 $, is \begin{array}{l} 14a-7=21 \\\\ 14a=21+7 \\\\ 14a=28 \\\\ a=\dfrac{28}{14} \\\\ a=2 .\end{array} CHECKING: \begin{array}{l} 7(2\cdot2-1)=21 \\\\ 7(4-1)=21 \\\\ 7(3)=21 \\\\ 21=21 \text{ (TRUE)} .\end{array} Hence, the solution is $ a=2 $.