Position Relevant in Image

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C++

Hello,
I want to find the objects or any perticular position in the image (2d or 3d)
suppose I have image of 3cars(x,y,z) parked and I want to know whats the distance between the cars(x and y) and how far is the car (z) from the camera if i know the distance between the car (x)

Any articles or code would be huge help
you can assume any formate of image its doesnt matter and you can assument number of camera up to 6 if one doesnt help.

Thank you very much guys in advance.