Add three to every number next as well as move up three letters.
For example: 1A is the starting point. To get from 1A to 4D you add three.
1 + 3 = 4 A to get to D is three letters up. A b c D
Same thing for 4D to get to 7G.
4 +3 = 7 D to get to G D e f G
Your answer: I'm not sure if you wanted me to find the letter of the equation or the actual next part. On your question you have just 13 up there, no letter so if youre trying to find the letter (13m) is your answer.
If you're trying to find the next equation to the puzzle (16p) is your answer.
