$a)\dfrac{-13}{24}+\dfrac{-5}{24}+\dfrac{7}{21}$
$=\dfrac{-13}{24}+\dfrac{-5}{24}+\dfrac{1}{3}$
$=\dfrac{-3}{4}+\dfrac{1}{3}$
$=\dfrac{-9}{12}+\dfrac{4}{12}$
$=\dfrac{-5}{12}$
$b)\dfrac{-5}{13}+\left(\dfrac{-8}{13}+1\right)$
$=\dfrac{-5}{13}+\dfrac{-8}{13}+1$
$=-1+1$
$=0$
$c)\dfrac{2}{3}+\left(\dfrac{3}{8}+\dfrac{-2}{3}\right)$
$=\dfrac{2}{3}+\dfrac{3}{8}+\dfrac{-2}{3}$
$=\left(\dfrac{2}{3}+\dfrac{-2}{3}\right)+\dfrac{3}{8}$
$=0+\dfrac{3}{8}$
$=\dfrac{3}{8}$
$d)\left(\dfrac{-3}{4}+\dfrac{5}{8}\right)+\dfrac{-1}{8}$
$=\dfrac{-3}{4}+\dfrac{5}{8}+\dfrac{-1}{8}$
$=\dfrac{-3}{4}+\left(\dfrac{5}{8}+\dfrac{-1}{8}\right)$
$=\dfrac{-3}{4}+\dfrac{1}{2}$
$=\dfrac{-3}{4}+\dfrac{2}{4}$
$=\dfrac{-1}{4}$
