A beam of light in air is incident on the surface of a rectangular block of clear plastic (n = 1.44). If the velocity of the beam before it enters the plastic is 3.00 · 108 m/s, what is its velocity inside the block?

using this formula

n = V1/V2 (you make V2 subject)

it becomes:

V2 = V1/n (put in your values and cal.)

Explanation:

refractive index

= velocity in first medium / velocity in the second medium

NB : there's other ways to find refractive index, depending on the values you were given