Collinear points lie on the same line. So, you must look for other points that lie on the line that connects A and N. If you start from A and continue along that line, you can see that, after N, you will meet points M and F.
So, those points are collinear with A and N.
The y-intercept represents the initial girth of the tree. Assuming that the X-axis represents time in this scenario (as it usually does) the y-axis, therefore, represents the girth of the tree. The y-intercept is located where X = 0. So if no time has passed then this must be the first measurement of the girth of the tree.
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