Pat has two sheets of large, square stamps. one sheet consists of $9$ stamps arranged in a $3$-by-$3$ square. (that's $3$ stamps by $3$ stamps, not $3$ inches by $3$ inches.) the other sheet consists of $36$ stamps arranged in a $6$-by-$6$ square. the stamps on each sheet are separated by perforated creases. all the stamps on both sheets are the same shape and size.

the total length of the creases on the $3$-by-$3$ sheet is $18$ inches.

how many inches of creases does the $6$-by-$6$ sheet have?