Simple answer for intersecting lines?

I am scratching my head trying to figure out how to display a XY pixel value for 2 intersecting lines.

I am working on a 2d graphical application that deals with lines or slopes.

> Both lines always intersect

> I have line "CD" which pixel values are known for both C and D

> I know the pixel location of A

> the application is dynamic in that I can grab point B and move it vertically (assuming that CD is somewhat horizontal)

> I am attempting to display the pixel values in which AB and CD intersect

Can anybody lead me in the right direction? I appreciate your help!