Answer
The possible widths of the banner should be no longer than 6.
Work Step by Step
Let w = widths of the banner
Write: $2w + 2(18) \leq 48$
$2w + 36 \leq 48$
Simplify:
$2w \leq 12$
Divide each side by 2:
$w \leq 6$
The possible widths of the banner should be no longer than 6.