Answer
$x=\frac{3}{a}$
Work Step by Step
Given that $2^a=5$ and $2^b=7$, we have
$$
\begin{aligned}
25^x & =64 \\
\left(5^2\right)^x & =64 \\
5^{2 x} & =64 \\
\left(2^a\right)^{2 x} & =64 \\
2^{2 a x} & =2^6 \\
2 a x & =6 \\
x & =\frac{3}{a} .
\end{aligned}
$$
so $x=\frac{3}{a}$
