First of all, you should know/notice that
[tex]125=5^3[/tex]
You should also know how negative exponents work:
[tex]a^{-m}=\dfrac{1}{a^m}[/tex]
So, if you wanted
[tex]5^x=125[/tex]
the answer would be x=3, but since you want 125 to be at the denominator, the answer is x=-3:
[tex]5^{-3}=\dfrac{1}{5^3}=\dfrac{1}{125}[/tex]
