`a) ( ( - 2 )/3 ) . 3/11 + ( ( - 16 )/9 . 3/11 )`
`= 3/11 . ( ( - 2 )/3 + ( - 16 )/9 )`
`= 3/11 . ( - 22 )/9`
`= ( - 2 )/3`
`b) ( 1/2 - 13/14 ) : 5/7 - ( - 2 /21 + 1/7 ) : 5/7`
`= ( - 3 )/7 . 7/5 - 1/21 . 7/5`
`= 7/5 . ( ( - 3 )/7 - 1/21 )`
`= 7/5 . ( - 10 )/21`
`= ( - 2 )/3`
`c) 4/9 : ( - 1/7 ) + 6 5/9 : ( - 1/7 )`
`= 4/9 . ( - 7 ) + 6 5/9 . ( - 7 )`
`= ( - 7 ) . ( 4/9 + 6 5/9 )`
`= ( - 7 ) . 7`
`= - 49`
