$b)\sqrt{(2\sqrt{5}-7)^{2}}-\sqrt{45-20\sqrt{5}}$
$=7-2\sqrt{5}-\sqrt{25-2.5.2\sqrt{5}+20}$
$=7-2\sqrt{5}-\sqrt{(5-2\sqrt{5})^{2}}$
$=7-2\sqrt{5}-(5-2\sqrt{5})$
$=7-2\sqrt{5}-5+2\sqrt{5}$
$=2$
$c)\dfrac{10\sqrt{6}-12}{\sqrt{6}-5}-3\sqrt{\dfrac{2}{3}}+\dfrac{15}{\sqrt{6}-1}$
$=-2\sqrt{6}-\sqrt{6}+3+3\sqrt{6}$
$=3$