Hi all, I'm working through some problems at the moment and I am a little stuck on an algebraic rearrangement of a coordinate geometry proof. The problem is as follows:

We have a line passing through two points A: (x1, y1) and B: (x2, y2) (where x1 doesn't equal x2). Let P: (x, y) be any point on the line. We can define the equation of the line as:

[1] ((y-y1)/(x-x1))=((y2-y1)/x2-x1))

Putting this into the form y = mx +c gives

[2] m= ((y2-y1)/(x2-x1)) ... I get this, but then it says that

[3] c= (((x2y1)-(x1y2))/(x2-x1)) ... I don't understand how we arrive at this expression from rearranging [1], could someone please help?

Thanks,

Adam