Expanding the fraction allows you to see and/or reduce the numerator and the denominator.
From multiplication, across the top we see:
z^3 x 5
across the the bottom we get:
35z x 4z^2
From expansion we get:
all of the z's at the top cancel the z's at the bottom. The five at the top reduces to 1, and the 35 at the bottom reduces to 7.
This leaves 1/7*4, which results to 1/28.