check the picture below.
if the length of the inner perimeter is "y", and the width of the inner perimeter is "x", then the inner perimeter is x + x + y + y, or 2x + 2y.
now, the outer length and width are y+16 and x + 16, so the perimeter is (y+16)+(y+16)+(x+16)+(x+16), or 2x+2y+64. How many more feet is longer? well, notice.
