Hi. It asks to calculate the focal length range of a projector of square pixel SXGA is from 4 to 12 m from the screen. It gives the diagonal size of the display (in mm) as well as the horizontal size (in m). The thing that am not sure about is how the focal length would be calculated with the screen dimensions and whether info about square pixels and SXGA would be required. I assume the range means to calculate the focal length at 12 then at 4 and take these values away from one another? Any help is appreciated. Thanks.