Minimum Thickness Of Plastic Physics: Minimum Thickness Of Plastic?

Physics: Minimum thickness of plastic? - minimum thickness of plastic

A thin plastic film (s) = 1.60 with a slit of a double slit illuminated by light of 640 nm. The central point on the screen, rather than the maximum is dark.

What is the (minimum) thickness of plastic?

Am I obligated to use the double slit and single slit equations to solve this problem? They are faced with the equation of thin films, is not it? :)