Help on front door and back door analysis

[Michael Duventre]

hello everyone!!! I'm a college student trying to understand the front door and back door approach theory and how to calculate them. I am a baby in this. If you can help me please explain things to me as simply as you can. And if you can please no abbreviations. THANK YOU IN ADVANCE EVERYONE!!! :)

THE FRONT DOOR APPROACH:

Potential Gross income - vacancy & collection + operating income = effective gross income - operating expenses = net operating income - annual debt service = before tax cash flows

***is this correct for the formula for the FRONT door approach?!?

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BACK DOOR APPROACH:

I don't even know where to begin on this. Help!!!