What comes next? 21da, 28do, 35ta, 42am, 56im, ?

Question

Question was given by my niece's first-grade teacher.

Answer 1

number part be 7*n, n=3,4, 5, 6...become 21, 28, 35,42, 49...

Hav no idea wot the straenj letters mite be

Answer 2

63mt? The numbers are multiples of 7 excluding 7*7 (49). The letters may be representations of digits, although there is inconsistency. Using this idea, we need the letters for 6 and 3. So we use 35ta and 56im to suggest that t=3 and m=6.

Answer 3

Given 21da, 28do, 35ta, 42am, 56im,
from given d=2, t=3, m=6
7*n

Where n=3,4,5,6,8,9
63mt