The sum of two numbers is 21. One number is three less that the other. Find the numbers.

If you look at clue #2 first then you see that if you know one of the numbers then the other number is 3 less than it, so if I let one of the numbers be x then the other is

x - 3. Now clue # 1 says the sum of the two numbers is 21 so

x + (x - 3) = 21

12+9=21 because 12-3=9 and 9+12 is 21