Let $n = k\rho^{a}a^{b}T^{c}$ where $\lbrack\rho\rbrack = \left\lbrack ML^{- 3} \right\rbrack,\ \lbrack a\rbrack = \lbrack L\rbrack$ and $\lbrack T\rbrack = \left\lbrack MT^{- 2} \right\rbrack$ Comparing dimensions both sides we get $a = \frac{- 1}{2},\ b = \frac{- 3}{2}$ and $c = \frac{1}{2}\ \therefore\eta = k\rho^{- 1/2}a^{- 3/2}T^{- 1/2}$ $= \frac{K\sqrt{T}}{\rho^{1/2}a^{3/2}}$