how to do this?

> I want to deduce n[v]=2*n-n[c]-2 from
> 
> n[e]=1/2*(3*n[v]+n[c])
> 	and
> n-n[e]+n[v]=1
> 
> Another words: I want to express n[v] in terms of other variables.
> 
> But solve does't work this for me.

You need to solve for the *two* variables:

(C1) n[e]=1/2*(3*n[v]+n[c]);

                                 3 n  + n
                                    v    C
(D1)                        n  = ---------
                             e       2
(C2) n-n[e]+n[v]=1;

(D2)                       n  + n - n  = 1
                            v        e

(C3) solve([d1,d2],[n[v],n[e]]);

(D3)           [[n  = 2 n - n  - 2, n  = 3 n - n  - 3]]
                  v          C       e          C