Answer :

[tex]a^{-n}=\frac{1}{a^3}\\-----------------------\\\\U^{-3}=\frac{1}{U^3}\\\\therefore\\\\\frac{1}{U^3}=\frac{1}{125}\ \ \ \ \ |inverse\ both\ sides\\\\U^3=125\\\\U=\sqrt[3]{125}\\\\\boxed{U=5}\ because\ 5^3=125[/tex]
the answer is: U =5

first, you take the U^-3 and put it as 1/U^3, that is because you cannot have a negative exponent, you MUST turn the exponent into a positive....
now you have 1/U^3 = 1/125...     now you find the uncubed version of 125::::::

the cube root of 125 is 5

so, when you have U^-3, you are looking at 5^-3..