I think it depends on the image you are using. Is the scale linear in both 
directions (vertical and horizontal)? Then it should be simple mathematics.

dX = X2 - X1 = e.g. 42
dLon = Lon2 - Lon1 = e.g. 21

LonPerPixel = dLon / dX = 0.5

That means, each pixel (in a linear scaled image!) represents a longitude 
step of LonPerPixel degrees. You do the same for LatPerPixel (with Y and Lat, 
of course) and then it should be pretty simple to get the X/Y coordinates of 
the third point. Just get the Lat/Lon differences between the third and the 
first or second point, divide the differences through LonPerPixel and 
LatPerPixel and you know how much pixels you have to add to your first resp. 
second point in order to get the coordinates of the third one!

However, this could fail if P1 and P2 are either horizontally or vertically 
on the same line (as one of the differences would equal zero) or the image 
is scaled in any other way.

Regards,

  Martin Matysiak
  homepage <https://martin-matysiak.de> | blog<http://blog.martin-matysiak.de>

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