The numbers in B A C K (5 2 6 12) are ahead of the alphabets in the order of +3 +1 +3 +1 (i-e B is 2nd letter, 2+3 = 5, A is 1st letter 1+1=2, C is the third letter, 3+3 = 6, K is the 11th letter 11+1=12).
Since we are converting a word into a code the above procedure whould work fine. In question we are asked to inverse the above procedure where we are given a code which needs to be worked into a word. So the pattern is reversed. The +3 +1 +3 +1 becomes -3 -1 -3 -1. So The 9th letter is the letter I, minus 3 letters gives us the letter 'F', 10th letter is J, -1 letter gives us 'I', 17th letter is 'Q', take away three letters and we get 'N', the 6th letter is 'F', minus 1 is 'E'.
We are doing code to word as opposed to the question where we are given word to code, we are expected to not only answer the question but answer it in a reverse mode, instead of adding 3s and 1s, we must take away 3 and 1 letters alternatively as we are converting a code into letters not letters into code.. Hope it makes sense!