3138 and 51804 are both multiples of 6.  However the fact that

it says "strictly between 3138 and 51804", we must begin with

the first multiple of 6 larger than than 3138, which is 3144.

Also we must end with largest multiple of 6 less than 51804,

which is 51798.  So,

, 

The formula for the nth term is

















The formula for the sum of the n terms 
of an arithmetic sequence is

  









Edwin