The little lines in each side show that the sides are the same length but you also need to find the length of the smaller side which isn’t the same. For this imagine that the shape is split into a square and a triangle and you need to find the long side of the triangle using Pythagoras
a^2 + b^2 = c^2
20^2 + 20^2 = 800
Square root of 800 = 28.3
Then do 28.3-20=8.3
So I think the answer is 20+20+20+20+20+8.3=108.3 cm