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