I am trying to simplify following expression using only Theorems and postulates of boolean Algebra.

x'y'z'w'+x'y'zw'+x'yzw'+x'yzw+xy'zw'+xy'zw+xyz'w+x yzw'

The answer is:

zw'+x'y'w'+x'yz+xy'z+xyz'w

Im having a spot of trouble simplifying a boolean algebra expression, I tried everything i can think of but nothing seems to look right. Only with map of Karnaugh I can get the result.