X and X' in affine transformations

Hello. I am working on 2D Affine transformations and I need to select 3 points from original image interactively with the mouse. Before finding Affine transformation matrix I need to find the X' the corresponding points in the second moved image with respect to original image. This X' is the same 3 points with different coordinates. Can anyone please help me how to find X'. Thank you in advance.