#### Answer

$b=3A-a-c$

#### Work Step by Step

$\bf{\text{Solution Outline:}}$
To solve the given equation, $
A=\dfrac{a+b+c}{3}
$ for $
b
,$ use the properties of equality to isolate the variable.
$\bf{\text{Solution Details:}}$
Using the properties of equality to isolate the variable, the equation above is equivalent to
\begin{array}{l}\require{cancel}
A=\dfrac{a+b+c}{3}
\\\\
3\cdot A=3\cdot \dfrac{a+b+c}{3}
\\\\
3A=a+b+c
\\\\
3A-a-c=b
\\\\
b=3A-a-c
.\end{array}