[tex]\bf -\cfrac{16}{21}-\left( -\cfrac{13}{15} \right)\implies -\cfrac{16}{21}+\cfrac{13}{15}\impliedby \textit{we'll use the LCD of 105}
\\\\\\
\cfrac{(-16\cdot 5)~+~(13\cdot 7)}{105}\implies \cfrac{-80~+~91}{105}\implies \cfrac{+11}{105}\implies \cfrac{11}{105}[/tex]
in case you wonder why we used 105 as LCD,
21 = 3 * 7
15 = 3 * 5
the "3" is common to both denominators, so we "use it once" and our LCD is then 3 * 7 * 5.
