Question

if a super dense material with a refractive index of 1.76 x10^7 what is the speed of light in this material

Answer 1

Using the formula n=c/v where n is the refractive index of a material, c is the speed of light and v is the speed of light within the material, you can manipulate the above formula to v=c/n to find the speed within the material.
*Thats the theory anyways, I see here that your index of refraction is huge, normally the index of fraction is a little above one. A perfect vacum would be 1.00, water is 1.33. A material has an index of refraction of 1.76E7 will slow light down to a very, very slow pace, then again, its a super dense material...*

Answer 2

There are indeed exotic materials (at near-absolute zero temperature) that slow light that much, but they are not "super dense". They are called Bose-Einstein condensates.