`#Kenshiro`
`a ) 10/21 xx` `\frac{-14}{3}` `+ 5/9`
`=` `\frac{-140}{63}` `+ 5/9`
`=` `\frac{-140}{63}` `+ 35/63 =` `\frac{-105}{63}` `=` `\frac{-15}{9}`
`b ) 7/8 xx 7/13 + 7/8 xx 9/13 - 7/8 xx 3/13`
`= 7/8 . (7/13 + 9/13 - 3/13 )`
`= 7/8. 13/13`
`= 91/104 = 7/8`