The class is about heat, waves, and light. The website wants exactly the perfect answer and in the same metric system units that are stated on the calculators that are on the picture for the question, if not, it'll mark it wrong.

Set 16 Problem 3 A very thin sheet of plastic ([1 = 1.5?) covers une slit of a double-slit apparatus illuminated by 626 -nm light. The center point an the screen, instead of being a maximum. is dark. What is the (minimum) thickness t of the plastic? The wave traversing the plastic must get exactly one-half wave uut-Dfphase with the wave traversing an equivalent distance in air