The bottom right.
The line best fit is usually the line that follows the trend of dots in the scatter plot. In the other 3, you can tell that there seems to be no trend between either the line or the dots.
The bottom right one has a line pretty much right through the middle of the scattered dots and follows the trend.
Amy would have spent $22.5 on her books (assuming you meant $10 total, not $10 per book)
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Angle b will be 34 degrees because angle b and the other angle that is 34 degrees are vertical angles!
