SH
In the above question, the pattern is,
1st letter
$latex A\xrightarrow{\text{+6}} G\xrightarrow{\text{+6}} M\xrightarrow{\text{+6}} \textcircled{S} \xrightarrow{\text{+6}} Y $
2nd letter
$latex Z\xrightarrow{\text{-6}} T\xrightarrow{\text{-6}} N\xrightarrow{\text{-6}} \textcircled{H} \xrightarrow{\text{-6}}B $