There are basically three types of

line equations:

1. y=mx+b

x and y represent a point on the line (x,y)
m represents the slope of the line
b represents the y-intercept
to find the y-intercept let x=0
to find the x-intercept let y=0

2. ax+by=c

x and y represent a point on the line (x,y)
-a/b represents the slope of the line
a, b, and c are constants
the y-intercept is c/b when x=0
the x-intercept is c/a when y=0

3. (y-y_{1})=m(x-x_{1})

x_{1} and y_{1} represent a point(x_{1},y_{1}) on the line
m represents the slope of the line
*usually the preferred formula for lines because if you know a point on the line and the slope you know the equation

Parallel lines have the same slope

Perpedicular lines have slopes that are negative reciprocals
e.g. if m is the slope of one line then the slope of the other line is -1/m

Slope can be calculated by using this formula:
(y_{2}-y_{1})/(x_{2}-x_{1})
*or is more easily known as rise/run