A straight line can always be written in the form of...
y = mx + c
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Drag "y-intercept" to change the y-intercept or c.
Drag "A" to change the slope or m.
Drag "1" to change the scale.

Figure out the equation of the straight line before clicking on the "Show Equation" button.  
Click "Hide",
 drag some points and repeat the exercise.

• What is the slope of a horizontal (flat) line?
• Drag the points to form the line y = 2.5
• What is the slope of a vertical line?