a) \(\dfrac{{ - 3}}{7} + \dfrac{5}{{13}} + \dfrac{{ - 4}}{7} \)
\(= \dfrac{{ - 3}}{7} + \dfrac{{ - 4}}{7} + \dfrac{5}{{13}}\)
\(= \dfrac{{ - 7}}{7} + \dfrac{5}{{13}} = - 1 + \dfrac{5}{{13}}\)
\(=\dfrac{-13}{13}+\dfrac{5}{13}=\dfrac{-8}{13}\).
b)
\(\eqalign{
& {{ - 5} \over {21}} + {{ - 2} \over {21}} + {8 \over {24}} \cr
& = \left( {{{ - 5} \over {21}} + {{ - 2} \over {21}}} \right) + {8 \over {24}} \cr
& = {{ - 7} \over {21}} + {8 \over {24}} \cr
& = {{ - 1} \over 3} + {1 \over 3} = 0 \cr} \)
