Question 970663
The width of the wrapping paper is needed as part of solving the question.  The box to be wrapped is described, and the "If..." part is not important.


21*9*15 volume using those dimensions will have a surface area:  {{{2(21*9+9*15+15*21)}}} square inches.  This is 1278 square inches surface area of the box.


The length to use of the wrapping paper depends on the width of the wrapping paper.  At least {{{1278* in^2}}} of paper will be needed.  Certainly MORE, because of the need to overlap and fasten with adhesive tape.