Answer
$7^{6}a^{2}b^{4}$
Work Step by Step
$(\frac{7^{-3}}{ab^{2}})^{-2}$
Using $\frac{1}{a^{n}}= a^{-n}$
$= (7^{-3}a^{-1}b^{-2})^{-2}$
$= (7^{-3})^{-2}(a^{-1})^{-2}(b^{-2})^{-2}$
Using $(a^{m})^{n} =a^{m \times n} $
$=7^{6}a^{2}b^{4}$