`13/37 xx 7/29 - 7/29 xx 6/37 - (-7/29) xx 31/37`
`= 13/37 xx 7/29 + 7/29 xx (-6/37) + 7/29 xx 31/37`
`= 7/29 xx (13/37 - 6/37 + 31/37)`
`= 7/29 xx 1`
`= 7/29`
__________________________
`13/37 . 7/29 - 7/29 . 6/37 - (-7/29) . 31/37`
`= 13/37 . 7/29 + 7/29 . (-6/37) + 7/29 . 31/37`
`= 7/29 . (13/37 - 6/37 + 31/37)`
`= 7/29 . 1`
`= 7/29`