Your book is wrong.
Now in the part where is says then it will equal 0. And you know that anything divided by 0 is undefined.
Now we multiply everything by giving us:
Cancel out like terms leaving:
And then divide everything by to get the answer:
Now we see the part where it says we know that it will be 0 because = 0. (I should have cancelled that out from the start...)
Can you carry on from there? Or am I wrong?