Answer
30 inches by 42 inches
Work Step by Step
perimeter = 144 inches, length is 12 inches longer than width
$2x+2y=144$
$x=12+y$
$2x+2y=144$
$2(12+y)+2y=144$
$24+2y+2y=144$
$4y=120$
$4y/4=120/4$
$y=30$
$x=12+y$
$x=12+30$
$x=42$