$$\cfrac { -6 }{ 13 } -\left( \cfrac { -7 }{ 15 } \right) =\cfrac {
-6\times 15 }{ 13\times 15 } -\left( \cfrac { -7\times 13 }{ 15\times 13
} \right) =\cfrac { -90 }{ 195 } -\left( \cfrac { -91 }{ 195 }
\right) =\cfrac { -90-(91) }{ 195 } =\cfrac { -90+91 }{ 195 } =\cfrac { 1
}{ 195 } $$
L.C.M of $$13$$ and $$15$$ is $$195$$