Groups and the Group Axioms · Easy problem

A dial lock works in the group $(\mathbb Z_{10},+)$: turning it by $a$ clicks then by $b$ clicks is the same as turning it by $a+b$ clicks mod $10$, and position $0$ means "unlocked". The dial has just been turned by $6$ clicks. How many more clicks $x$ (with $0\le x<10$) are needed to return it to position $0$?