To solve, apply the formula of index of refraction which is:
n - index of refraction
c - velocity of light in a vacuum ( 3 x 10^8 m/s) and
v - velocity of light in a medium
So, plug-in n=2.25 and c=3 x 10^8 to the formula.
Then, multiply both sides by v.
`2.25*v=(3xx10^8)/v * v`
And, divide both sides by 2.25.
Hence, the velocity of light in the medium is `1.33xx10^8` m/s.