Answer
$x=\frac{2}{3}$
Work Step by Step
We know that if $a\gt0,a\ne1$, then $a^b=a^c\longrightarrow b=c$
$25=5^2, 125^x=(5^3)^x=5^{3x}$
Thus here: $3x=2\\x=\frac{2}{3}$
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