1. X—>Y Premise 2. (Y v ~X) —> (Y—>Z) Premise 3. Y v ~X 1, Material Implication (MI) [ (A—>B) <—> (~A v B) and (~A v B ) <—> (B v ~A) ] 4. Y—>Z 3,2 Modus Ponens (MP) 5. :: ~Z Condition Proof (CP) assumption 6. :: ~Y 5,4 Modus Tollens (MT) 7. :: ~X 6,3 Disjunctive Syllogism (DS) 8. ~Z —> ~X 5,7 CP (done)