So all around is 360 degrees. The sides directly in front of each other are congruent.
(5z+8) + (4z+20) + (5z+8) + (4z+20) = 360
There is two of each one because of the congruent sides.
Combine like terms.
18z + 56 = 360
Get the variable by itself.
18z + 56 = 360
-56 -56
18z=360
divide by 18
z = 20
Hope this helps! If you have any questions, just ask!
