The "line of best fit" is the result of a linear regression computation. If you have done that, you already know its equation.
If you have "eye-balled" the line into place, then you have to figure out its equation from points you read on the graph. We take a couple of the points to be ... (2001, 1700) and (2010, 2450)
The line through these points will have the equation
... y = (2450 -1700)/(2010 -2001)(x -2001) +1700
... y = (250/3)(x -2001) +1700
Putting x=2013 into this equation, we get ...
... y = (250/3)(2013 -2001) +1700
... = (250/3)(12) +1700
... y = 2700
The population is predicted to be 2700 in 2013.
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Had you drawn your graph with a vertical grid line at 2013, you could have read the number from the graph.
