



 
The area of a rectangle is the length times the width. You have a length of $w  15$ units and a width od $w + 15$ units and hence the area is $(w  15) \times (w + 15)$ square units. But the problem states that the area is 864 square units and thus \[(w  15) \times (w + 15) = 864.\] Expand the left side and solve for $w.$ Penny  


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