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Consider the case of going from point A at (20,40) to B at (100,100). Look at the difference (or change) along the x-axis. It went from 20 to 100, an increase of 80. Look at the differece (or change) along the y-axis: it went from 40 to 100, an increase of 60.

The slope of the line is therefore 60/80 (change in y over the change in x). To apply this slope (in order to extend the line from B), apply these changes to B at (100,100). Our new point A will be (100+80, 100+60) = (180,160).

The same strategy may be applied to find a new point for C. Compare C(50,160) to the vertex B(100,100). x increased by 50, but y decreased by 60. The new co-ords for C will be (100+50, 100-60), that is, C'(150,40).

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