Math Question

Math:
For any number W and Z, W.Z=W^3Z(8-W^2). If both Z and W.Z are positive numbers, which of the following could be a value of W?
1) 9
2) 3
3) 0
4) -2
5)-9



Thanks
* And, please tell me what "W.Z" mean mathematically.
更新: Thanks for your reply. And YES I can read Chinese. English is easier for me.
I have the same book you refer to and I also found it illogical in the book. (Answer key)
Well, I thought I misunderstood "W.Z" and that led me to the wrong answer.
But thanks anyway.
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