You didn't say how they were cast in tableau -
If they are strings then you can use Left((snap_date),2) and get the week and right((sanp_date),4) (or 2)
they will be strings that can be concatenated using [week]+'-'+[year] as a string
BTW if it came as an number you can recast it as a string with str(snap-date)
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you can use the DATENAME function for this.
for 242018 you can use
DATENAME('week', [SNAP_DATE] ) + DATENAME('year', [SNAP_DATE] )
for 2418 you can use
DATENAME('week', [SNAP_DATE] ) + RIGHT( DATENAME('year', [SNAP_DATE] ),2 )
Thank you ! This is exactly what I was looking for.