`a, (-6)/18 + (- 5)/30 - 7/9 + 7/21 + (-5)/6`
`= (-1)/3 + (-1)/6 - 7/9 + 1/3 + (-5)/6`
`= ((-1)/6 + (-5)/6) + ((-1)/3 + 1/3) - 7/9`
`= -1 + 0 - 7/9`
`= (-16)/9`
`b, (-21)/25 × 7/42 + 16/42 × 21/25`
`= 21/25 × ((-7)/42 + 16/42)`
`= 21/25 × 3/14`
`= 9/50`