So if he is taking a 200 milligrams tablet three times a day, you first want to multiply 200 by 3.
200 · 3 = 600
Then you want to take the 600 and and multiply it by 21, for the 21 days he took 600 milligrams of prescription.
600 · 21 = 12,600
Your answer is 12,600 milligrams.
Hope this helps! :)