Give reasons for the steps, given premises 1, 2 and 3:
1.  X ⊃ (Y ⊃ Z)
2.  X ⊃ (A ⊃ B)
3.  X • (Y ∨ A)     /∴  ∼B ⊃ X 
4.  X                 3, Simplification
5.  A ⊃ B           2,4, Modus Ponens
6.  Y ⊃ Z           1,4, Modus Ponens
7.  (Y ∨ A) • X       3, Commutation
8.  Y ∨ A             7, Simplification 
9. ∼(∼Y) ∨ A          8, Double Negation
10. ∼Y ⊃ A           10, Material Implication
11. ∼Y ⊃ B         10,5, Hypothetical Syllogism
12. ∼B ⊃ ∼(∼Y)       11, Transposition
13. ∼B ⊃ Y          12, Double Negation
14.  X ∨ ∼Y           4, Addition
15.  ∼Y ∨ X          14, Commutation
16.  Y ⊃ X           15, Material Implication
17. ∼B ⊃ X        13,16, Hypothetical syllogism
Edwin