#### Answer

$a=2$

#### Work Step by Step

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
$.