$\text{For ray to emerge parallel to water surface angle of incidence should be critical angle }$

$\Rightarrow r = C = sin^{-1}\frac{1}{\mu_w} = sin^{-1}\frac{3}{4}$

$\text{Apply snells law of water - glass interface }$

$\Rightarrow \mu_g sin i = \mu_w sinr =1$

$\Rightarrow \mu_g = \frac{1}{sini}$