A transparent, non-magnetic medium has a dielectric constant of...

For a non-magnetic medium, the relative permeability is approximately unity ($\mu_r = 1$).

The speed of light in a material medium is given by the relation:
$v = \dfrac{c}{\sqrt{\mu_r \epsilon_r}}$

Substituting the given values ($\epsilon_r = 2.25$, $\mu_r = 1$, $c = 3 \times 10^8 \text{ m/s}$):
$v = \dfrac{3 \times 10^8}{\sqrt{1 \times 2.25}}$

$v = \dfrac{3 \times 10^8}{1.5} = 2 \times 10^8 \text{ m/s}$

Answer: $2 \times 10^8 \text{ m/s}$