book Maths Puzzle Numerical Ability

Can someone help me simplify the following problems

(-3x / 2y^3)^-2

and

(x^1/3 / x^5)^3

thanks

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book Other Question

Mr. Turner invested a sum of money at 7% and twice that sum at 3%. His yearly return was $390. How much did Mr. Turner invest at each rate?
(This problem is not from a textbook). Please help, Thank You I've forgotten how to go about adding and subtracted negatives numbers. Can you help me with what rules to follow? Given the following eight problems:

+25 - (+10)
+25 - (-10)