(b) simplify the $\dfrac{3}{15}$ in the denominator first, then multiply numerator and denominator by the common denominator of all the fractions in the complex fraction ...
$\dfrac{10}{10} \cdot \dfrac{\frac{2}{5}+\frac{1}{2}}{\frac{1}{10}+\frac {1}{5}} = \dfrac{4+5}{1+2} = \dfrac{9}{3} = 3$
now, you try the same method with (a)
$\sqrt{45} = \sqrt{9 \cdot 5} = \sqrt{9} \cdot \sqrt{5} = 3 \sqrt{5}$