Given the input:

```
double x1,y1,x2,y2;
```

How can I find the general form equation (double a,b,c where ax + by + c = 0) ?

Note: I want to be able to do this computationally. So the equivalent for slope-intercept form would be something like:

```
double dx, dy;
double m, b;
dx = x2 - x1;
dy = y2 - y1;
m = dy/dx;
b = y1;
```

Obviously, this is very simple, but I haven't been able to find the solution for the general equation form (which is more useful since it can do vertical lines). I already looked in my linear algebra book and two books on computational geometry (both too advanced to explain this).