Try this,
declare @wk int set @wk = 21
declare @yr int set @yr = 2016
select dateadd (week, @wk-1, dateadd (year, @yr-1900, 0)) - 4 -
datepart(dw, dateadd (week, @wk-1, dateadd (year, @yr-1900, 0)) - 4) + 1
or try this way
declare @wk int = 21
select dateadd(week,@wk-1, DATEADD(wk, DATEDIFF(wk,-1,DATEADD(yy, DATEDIFF(yy,0,getdate()), 0)), 0))
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