Calculating the new x position

Hi ive been looking on the internet for a post from 1999 about 3d graphics algorithms if anyone could link to that it would be great and would solve the entire issue.

here is the issue.
Say I have my camera and x1 y1 z1 and i have a vector (just a pixel) and x2 y2 z2. How would I figure out the position I draw the x and y if Im drawing it orthagonally?