To find out if it's possible using only plus or minus we have 8 possibilities:
22+9+8+5=44
22+9+8-5=34
22+9-8+5=28
22+9-8-5=18
22-9+8+5=26
22-9+8-5=16
22-9-8+5=10
22-9-8-5=0
So it's not possible to make 6 if the numbers can only be used once.
If they can be used more than once we can use 16-10=6 or 34-28 so:
(22-9+8-5)-(22-9-8+5)=(22+9+8-5)-(22+9-8+5)=6.
But these are the same as 8+8-5-5, and 22 and 9 aren't used.
We can also use 26-10-10=(22-9+8+5)-(22-9-8+5)-(22-9-8+5)=6 which is the same as 9+8+8+8-5-22=6.