The point of contact the path difference is zero but one of the interfering ray is reflected so the effective path difference becomes λ/2 thus the condition of minimum intensity is created in the center.

Explanation
the density of an object is given by:

Step 1
find the volume of the bar
a)find the volume of the rectangular bar.
the volume of a rectangular prism is given by:

replace

Step 2
now,
Let

replace in the formula

therefore, the answer is

I hope this helps you