I'm not sure, but tell me if this looks right to ya.

m2 is meters squared

Back wall = 1.7m2
Front wall = 1.7m2
left side = 1m2
right side = 1 m2

Total surface area = 5.4m2

mylar sheets = 1.4m2

5.4m2/1.4m2 = ~4 sheets