Depends on how the papers are connected. if they are connected with the 7cm side joining each other, use solution one. if they are connected with the 9cm side joining each other, use solution two.
perimeter of edges of sign= 2 [(9x3)+7] = 68 inches
left over= 144-68 = 76 inches
OR
perimeter of edges of sign= 2[(7x3)+9] = 60 inches