One form of the equation of a straight line is called the slope-intercept form because it contains information about these two properties.

The equation of a straight line

Any equation of the form y = mx + c where m and c are fixed numbers, (i.e. constants), has a graph which is a straight line.

For example, y = 3x + 5,y = \frac{2}{3}x + 8 have graphs which are straight lines.

The slope and intercept of a straight line In the equation y = mx + c the value of m is called the slope, (or gradient), of the line. It can be positive, negative or zero. Lines with a positive gradient slope upwards, from left to right. Lines with a negative gradient slope downwards from left to right. Lines with a zero gradient are horizontal. y x this line has a positive gradient this line has a negative gradient the gradient of this line is zero.