Precalculus (10th Edition)

$x=-2$
$5^{-x}=25\\ 5^{-x}=5^2$ We know that if $a^b=a^c$ and $a\ne0,1,-1$, then $b=c.$ Hence, $5^{-x}=5^2\longrightarrow -x=2$ Multiply both sides by $-1$ oto obtain: $-x(-1)=2(-1)\\ x=-2$.