Make the shape into two polygons. A rectangle and a triangle.
Find the area of the rectangle by multiplying lxw.
14x18=252
Then find the area of the triangle
A=l x w/2
20-14=6
18-11=7
6 x 7 = 42
42/2=21
252+21=273
273/5 is about 54
He can fit 54 chickens.