Question 1206984
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She's right. It was a typo I made in step 4. 
I had a 2 there that should have been a 1. 
I fixed it, in red below"

1. D ⊃ E
2. D v A
3. ~E                           / A

4. ~D      <font color="red"><b>1</b></font>,3, modus tollens
5. A       2,4, disjunctive syllogism  

Edwin</pre>