Solution to 4.

 

1. ~k->~m
2. ~m->l
3. k->~l /.:k<->m
4. m->k 1 trans
5. ~l->m 3 trans
6. k->m 3,5 hs
7. (k->m)&(m->k) 4,6 conj
8. k<->m 7 be