For a given function, slope is defined as the change in outputs, or y-values divided by the change in inputs, or x-values. In essence the slope asks "For a given change in x, how much does y change?" or even more simply: "How steep is the graph of this function?". This can be represented mathematically by the formula:

Since we have a table of x,y pairs it's the last form of that equation that will be the most useful to us. To compute the slope we can use any two pairs, say the first two, and plug them into our formula:

We can check this answer by using a different pair, say the last two:

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As a common sense check: Our y-values get smaller as our x-values get bigger so a negative slope makes sense.
m=-3
14000+7x=35x (x being the number of books) solve for x and you have your answer. 14000=35x-7x=28x therefore x=14000/28=500 so he needs to sell 500 books before he breaks even.
.21 as a fraction would be 21/100
B, because you would just assume the line continues forever on both sides.