A biologist keeps a specimen of his favorite beetle embedded in a cube of polystyrene plastic. The hapless bug appears to be 2.0 cm within the plastic.
You forgot to give the refractive index for the plastic .. assuming it is 1.60
For overhead viewing .. n = actual depth / apparent depth
actual depth d = n x app. depth = 1.60 x 2.0cm .. .. .. ►d = 3.20 cm