The length $l,$ width $w,$ and height $h,$ of a rectangular carton are $l=12$ feet, $w=4$ feet, and
$h=3$ feet. The length of a diagonal, $d$ , the distance from one corner of the box to the opposite corner, is given by the formula $d^{2}=l^{2}+w^{2}+h^{2} .$ What is the length of the diagonal of the carton?