$$\cfrac { 5 }{ 63 } -\left( \cfrac { -6 }{ 21 } \right) =\cfrac {
5\times 1 }{ 63\times 1 } -\left( \cfrac { -6 }{ 21 } \right) =\cfrac {
5\times 1 }{ 63\times 1 } -\left( \cfrac { -6\times 3 }{ 21\times 3 }
\right) =\cfrac { 5 }{ 63 } -\cfrac { -18 }{ 63 } =\cfrac { 5-(-18) }{
63 } =\cfrac { 5+18 }{ 63 } =\cfrac { 23 }{ 63 } $$
L.C.M of $$63$$ and $$21$$ is $$63$$