If the boy is 3 feet tall and his shadow is 7 feet long, that is a dilation (increases in size) so you would divide 3 by 7 to get your rate of dilation. 3/7 is about 0.428. So if you want to know how tall the man is who casts a 14 feet long shadow is, you’d have to multiply 14 by .428 which you give you 5.992 which is about 6 feet tall.
Hope this helps