Thanks so much for that, I managed to work it out by myself in my head as I was out for a jog but thanks anyway.

The next bit has me stuck though as it doesn't seem to cancel down correctly to the given answer.

I'm told to use a similar method for 'z-1' (meaning

)

So I write it out in the same way as:

This is my working:

=

=

=

Stop right there! You're almost finished at this point.
=

Then try to find the modulus by squaring both terms:

However the answer given in the book is

, which the terms above doesn't cancel to. Can anyone see where I've gone wrong?