The equation to any straight line is
y = mx + c ...........(1)
By properly determining the quantities m and c we can make (1) represent any straight line we please.
If (1) pass through the point (x', y'), we have
y' = mx' + c .......(2)
Substituting for c from (2), the equation (1) becomes
y - y' = m(x - x') ........(3)
This is the equation to the line going through (x', y') making an angle tan-1 m with OX. If in addition (3) passes through the point (x", y"), then
Substituting this value in (3), we get as the required equation
