\(-\dfrac{6}{13}.\dfrac{13}{21}+\dfrac{6}{13}.-\dfrac{8}{21}\)
\(=\dfrac{6}{13}.\dfrac{-13}{21}+\dfrac{6}{13}.\dfrac{-8}{21}\)
\(=\dfrac{6}{13}.\left(\dfrac{-13}{21}+\dfrac{-8}{21}\right)\)
\(=\dfrac{6}{13}.\dfrac{-21}{21}\)
\(=\dfrac{6}{13}.\left(-1\right)\)
\(=\dfrac{-6}{13}\)
`-6/13 . 13/21 + 6/13 . -8/21`
`= -6/13 . 13/21 - 6/13 . 8/21`
`= -6/13 . (13/21 + 8/21)`
`= -6/13 . 21/21`
`= -6/13 . 1`
`= -6/13`