Answer
The width of the frame is $2$ inches.
Work Step by Step
With $ x $ being the width of the frame, the outer length equals $16+2x $ (two frame widths added, one on either side), and the outer width equals $12+2x $
Perimeter $=2\times($ length + width)
$=2(16+2x+12+2x)$
$=2(28+4x)$
$=56+8x $
Given that $56+8x=72,\qquad $ ... subtract 56
$8x=16$
$ x=2$ inches
The width of the frame is $2$ inches.
