If you look at the picture, the graph shows the 8 turn into a 6 on the top of the polygon. Converted into a fraction, that is a 1/4 reduction. 1/4 of 10= 2.5.
Subtract that from 10.
10- 2.5= 7.5
z= 7.5
Hope this helps!
Brainliest answer is always appreciated!