Simplify each expression. Use only positive exponents.
(2x^-5 y^4)^3

(2x-5y4)3

Make sure every factor, including the 2, inside the 
parentheses shows an exponent.  So give the 2 its
understood exponent of 1.

(21x-5y4)3

Now, remove the parentheses by multiply each of the 
three inside exponents by the outside exponent 3

21·3x-5·3y4·3

23x-15y12

Now put this over 1

 23x-15y12
-----------
   1

and use the rule. 

To change the sign of the
exponent of a factor of the numerator, move
the base and the exponent from numerator to
denominator, and change the sign of the
exponent only:

and vice-versa:

[To change the sign of the
exponent of a factor of the denominator, move
the base and the exponent from denominator to
numerator, and change the sign of the
exponent only]

But you need only the first rule to bove the
x-15 to the bottom as x15

 23y12
------
  x15

Now you can replace the 25 by 8, since
2·2·2 = 8

 8y12
------
  x15

Edwin