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Let's plot the point: Now let's draw the horizontal line through it: Now let's plot another arbitrary points on that line: The arbitrary point on that line I chose is (2,5). Question: What do those two points have in common? Answer: They have the same y-coordinate, which is 5. Let's plot another point on that line: The arbitrary point on that line I chose this time is (-8,5). Question: What do those three points all have in common? Answer: They all three have the same y-coordinate, which is 5. Do you see that no matter how many arbitrary points I pick on that line, every one of them will have the same y-coordinate 5? Since that is true for every point on that line, the equation of that line could be the sentence: "The y-coordinate of every point on that line equals 5" But we just shorten it to simply y = 5 That's the equation of that horizontal line. Similarly every horizontal line has a very simple equation "y = some number" Edwin