Refractive index (symbol n) is defined as the ratio of the velocity of light in a vacuum (symbol c) to the velocity of light in a medium (symbol v). The equation is n=c/v. In order to obtain the speed of light in carbon disulphide the above equation needs to be rearranged to make v the subject. This yields v=c/n. Plugging the numbers in yields a speed to 1.84x10^8 metres per second.
