This is a puzzle with two clues.
1. The sum of two numbers is 21.
2. One number is three less than the other.
You might look at clue #1 first and see that there are two numbers you don't know and decide to call one of them x and the other y and then clue #1 is x + y = 21. I would rather not do this as it involves using two unknowns.
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
Solve for x.