If you're really challenged figuring this out, you can count out 21 of some object—such as pennies, scraps of paper, or even cookies—and deal them into piles of 3 each. Then count the piles. Working with actual physical objects can often give you mathematical insight not obtainable any other way.
Each pile subtracts 3 from the original amount. We use the arithmetic operation "division" to simplify repeated subtraction.
Knowing number facts such as 3×7 = 21 is very helpful to solving problems of this nature.
