Answer
$7|b|$
Work Step by Step
Using $\sqrt{x^2}=|x|$, the given expression, $
\sqrt[4]{(7b)^4}
,$ simplifies to
\begin{array}{l}
|7b|
\\\\=
7|b|
.\end{array}
We put the variable in absolute value bars since it is being raised to an odd power.