$\dfrac{\sqrt{7}}{\sqrt{20}}+\dfrac{\sqrt{1}}{\sqrt{35}}-\dfrac{\sqrt{81}}{\sqrt{140}}$
$=\dfrac{\sqrt{7}}{2\sqrt{5}}+\dfrac{\sqrt{35}}{35}-\dfrac{9}{2\sqrt{35}}$
$=\dfrac{\sqrt{7}.\sqrt{5}}{2.5}+\dfrac{\sqrt{35}}{35}-\dfrac{9\sqrt{35}}{2.35}$
$=\dfrac{\sqrt{35}}{10}+\dfrac{\sqrt{35}}{35}-\dfrac{9\sqrt{35}}{70}$
$=\dfrac{7\sqrt{35}+2\sqrt{35}-9\sqrt{35}}{70}$
$=\dfrac{0}{70}$
$=0$