Chapter R - Section R.2 - Real Numbers and Their Properties - R.2 Exercises - Page 19: 113

Both $x$ and $y$ need to be positive OR both $x$ and $y$ need to be negative.

Both x and y need to be positive OR both x and y need to be negative for the statement to hold true. x-cubed is positive when x is positive and x-cubed is negative when x is negative. So, only when x cubed and y have the same sign for this statement to become positive.