Question 1104279
{{{(7-3(k+1))-(7-3k)}}}
{{{(7-3k-3)-(7-3k)}}}
{{{(-3k+4)-7+3k}}}
{{{-3k+4-7+3k}}}
{{{-3}}}
This {{{-3}}} looks like a common difference for any k being a whole number.  Probably works for integers.