1.      I got the same answer as yours:
P = 2 (15 + w)
2.      I also got the same as yours:
P = 2 (15 + w) ≤46
3.      To solve for the width we have to leave the w to the left side alone like this:
15 + w ≤ 46/2
W ≤ 23-15
W ≤ 8 feet
4.      The width of the banner is less than or equal to 8 feet.
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