The greatest common factor is 2z^2
How you find it is first you find the greatest common factor of the two numbers, which is 2 (2 is the greatest number that can be multiplied into 2 and 10; 2 times 1 = 2 and also 2 times 5 = 10)
Then you find the greatest common factor of the variables, which is z^2.
(z^2 is the largest that can go into z^3 and z^2 because z^2 times z = z^3 and z^2 = z^2)
After that, you just multiply them together to get 2z^2