Some real estate economists have argued that anchor stores (such as Macy's and Bloomingdales) in shopping malls create significant externalities for overall sales

In order to complete this assignment, I recommend that you read some articles on this subject. I found a couple on the Internet.

CONTRACTS, EXTERNALITIES, AND INCENTIVES IN SHOPPING MALLS by Eric D. Gould, B. Peter Pashigian, and Canice J. Prendergast is available at
http://economics.huji.ac.il/facultye/gould/gould_pashigian_prendergast_restat2005.pdf
The article discusses how anchor stores attract sufficient business as to justify lower rents. "On average, anchor stores occupy over 58% of the total leasable space in the mall and yet pay only 10% of the total rent collected by the developer," the article reports. But because the mall developer reaps additional rent when sales reach a certain threshold, this is a good deal for everyone involved. This type of externality could be considered a positive externality: the anchor store generates business for the other stores, without any cost to itself.

SHOPPING CENTERS: CROSS-STORE PATRONAGE, COMPARISON SHOPPING, AND INTERNALIZING EXTERNALITIES by Charles C. Carter is available at http://www.kreaa.org/AsRES/doc/Carter%20Charles(D4).doc
This article discusses how shopping malls can internalize the positive demand externalities with different store arrangements, and should be helpful in answering the second part of your question.