Side AB is on the y-axis, and having the same x-coordinate, has a length that is the ABSOLUTE difference between the y-coordinates. Therefore, AB = |0 - 6| or |6 - 0| = 6 units

Side AC is on the x-axis, and having the same y-coordinate, has a length that is the ABSOLUTE difference between the x-coordinates. Therefore, AC = |0 - 4| or |4 - 0| = 4 units

DE = 3 units in the smaller right-triangle CORRESPONDS to AB in the larger right-triangle, and so, it's obvious that DE is  of side AB. So is side DF which corresponds to AC and is also  of AC, or 

Within what I just stated, you should be able to answer the question, "why?" 

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