select * from task_reg t
where [date]=(select max([date]) 
				from task_reg tt 
			   where tt.ac=t.ac and tt.tasknumber=t.tasknumber)
and ac=18

WITH CTE AS
(
 SELECT N=ROW_NUMBER()OVER(PARTITION BY ac,tasknumber ORDER BY [date] DESC),*
 FROM task_reg
)
SELECT * FROM CTE WHERE N=1;