$\begin{array}{l} b)\\ \sqrt {{{\left( {\sqrt 5 - 3} \right)}^2}} - \sqrt {\dfrac{1}{5}} \\ = \sqrt {{{\left( {3 - \sqrt 5 } \right)}^2}} - \dfrac{1}{{\sqrt 5 }}\\ = \left| {3 - \sqrt 5 } \right| - \dfrac{{\sqrt 5 }}{5}\\ = 3 - \sqrt 5 - \dfrac{{\sqrt 5 }}{5}\\ = \dfrac{{15 - 6\sqrt 5 }}{5} \end{array}$