not sure how to do this

Answer:
(Do the inside first)
1) z-{2z-[3z-(4z-5z)-6z]-7z}-8z
2) z-(2z-(3z- (-1z)-6z)-7z)-8z
3) z-(2z- (-2z) -7z) -8z
4) z-(-3z)-8z
= -4z

I think, double check could be wrong since i rushed it. Hope this helps.