prove a lemma sum of all elements of a list l is equal to n, (sortascend n) is equal to n

Question

following is the one I\'m having trouble with.

Lemma  sum_asc_dec_eq : forall k1 n , 
 addmem_list( k1) = n -> 
 addmem_list( sort_des k1) = n 
->  addm