A line either slopes up or down or is horizontal. And, sometimes it is so steep that it is vertical but in that case it is not a function (x = some constant value).[br][br]A line slopes upward if when moving from left to right the values of the line increase, i.e., go up.[br]A line slopes downward if when moving from left to right the values of the line decrease, i.e., go down.[br][br]When a line is horizontal, the y-values do not change. This means that y = a constant value.

Drag the points A and B to change the position of the line. Note how the gradient changes. The denominator is the change in x-direction and the numerator is the change in y-direction.[br][br]To determine the whole equation of the line you also need the [b]y-intercept [/b]where the line intersects the y-axis. The y-intercept is the y-coordinate of the point of the line where it intersects the y-axis.[br][br]Together with the gradient and the y-intercept, your line equation is y = [b]gradient[/b] times x + [b]y-intercept[/b]