y = -
x +

The equation of a line in slope-intercept form is
y = mx + c ( m is the slope and c the y-intercept )
rearrange 6x + 12y = 9 into this form
subtract 6x from both sides
12y = - 6x + 9 ( divide all terms by 12 )
y = -
x +
( which simplifies to )
y = -
x +
