3 points (a,b) (c,d) and (e,f)

I need to know if point (e,f) lies on the line passing through (a,b) and (c,d)

I know that is the slope of all the lines is the same then they all lie on the same point... but I'm doing that and it's not working!

if you can do that... then how about this...

Take the line through (a,b) and (c,d) and draw a line perpendicular to it at point (a,b).

Now find if point (e,f) lies on the same side of the perpendicular line as (c,d) or not!

These are both for a PC program I'm writing, and my maths really isn't up to scratch! I'm getting a headache and could really do with some help! Please bear in mind, the answer needs to be "zero proof", as in, the computer can't devide by zero, so it needs to avoid that...

any help muchly appreciated!