Start out assuming the negation of the conclusion and reach a contradiction. The conclusion is ~A, so its negation is ~~A A contradiction is always in the form p & ~p. It's C & ~C. We reach it in step 10. 1. (A & B) ⊃C 2. B & ~C/~A | 3. ~~A Assumption for Indirect Proof | 4. A 3, Double Negation | 5. B 2, Simplification | 6. A & B 4,5, Conjunction | 7. C 1,6, Modus Ponens | 8. ~C & B 2, Commutation | 9. ~C 8, Simplification |10. C & ~C 7,9, Conjunction 11. ~A Lines 3-10 for Indirect Proof Edwin