Slope Intercept Form
If a line has slope m and y-intercept b, then the equation of the line in slope-intercept form is
y = mx + b.
when b = 0, we say that y is proportional to x.
If a line has slope m and passes through the point (x1 , y1), then an equation of the line is given by
y – y1 = m(x – x1) ,
the point-slope from of the equation of a line.
The slope of a horizontal line is 0.
The slope of a vertical line is undefined.