Given Function is f(x,y,z)=x'y'z+x'yz+xy'z+xyz
Now what is dual?
The dual of a boolean expression is obtained by interchanging boolean sums and products and interchanging 0's and 1's.
So dual of given function f(x,y,z)=(x'+y'+z)(x'+y+z)(x+y'+z)(x+y+z)