I multiplied both sides of the equation by (z^2+1), this is because:
(z^2+1)(z^4-z^2+1) = z^6 +1
Which becomes easier to deal with.
NOTE: If you've done Series in 2U (if you haven't ignore), then notice that the sum 1 - z^2 + z^4 is a geometric series of ratio (-z^2) therefore we can use the...