They're all 2 more than a multiple of 3, so we subtract 2 from each

3, 12, 27, 48, 75

Divide each by 3

1, 4, 9, 16, 25

That's the perfect squares, so the sequence 5, 14, 29, 50, 77, …

has general term





We set that greater than or equal to 1000






Divide thru by 3





So 19 is the smallest integer for which the
sequence has 4 digits.

So we substitute n=19 in the general term




Edwin