Find and write down two numbers, the sum of which is 21 and the difference is 3.

Let us denote the required numbers by x and y.
According to the condition of the problem, the sum of these two numbers is 21, therefore, the following relationship takes place:
x + y = 21.
It is also known that the difference between these two numbers is 3, therefore, the following relationship holds:
x – y = 3.
We solve the resulting system of equations. Adding these two equations, we get:
x + y + x – y = 21 + 3;
x + x = 24;
2 * x = 24;
x = 24/2;
x = 12.
Substituting the found value for x into the first equation, we get:
12 + y = 21;
y = 21 – 12;
y = 9.
Answer: the required numbers are 12 and 9.