Explanation:
6n+3 gives the value of the nth term of a sequence, so if 1008 was actually a term in the sequence, 6n+3=1008 would evaluate to give a whole number for n:
6n+3=1003
6n=1000
n=16.6667
Since this is not a whole number, we know 1003 isn't a term for the sequence, rather it is inbetween the 16th and 17th term of the sequence