I assume that by square corners you mean 90 degree angles. if this is the case then there are 2 in a pentagon. Hope I helped :)
Answer:
Answer is x= 15
Step-by-step explanation:
(3x+18)+(8x-3)=180
11x+15=180
11x=165
x=15
72cm
you multiply the outside length (6cm each) by how many there are