By looking at the third octet it's 200 to 207 which means you have eight 3rd octet numbers. You need that mask where the 0s are 8 long so 255-8=247. But you need to include 255 which means you start at 248. 248 - 255 is exactly 8 long so your subnet mask for the third octet is 248.
Your full mask is 255.255.248.0. However, that doesn't mean that the desired range of 200-207 is a legitimate range so we must do a quick check. Fire up the windows calculator and type in 207, hit the AND button, type 248, and your result should be 200 which is the legitimate network ID and starting point for our subnet range.
248 in the third octet should be a /21 subnet mask. It would serve you well if you memorized the table of common subnet lengths and there's really not that many.
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