Find the value of two numbers if their sum is twelve and their difference is 4. explain to me how to do this and what the answer is!

So,

All you have to do is to translate the words into open sentences with placeholders.

Apparently, there are 2 numbers, call them x and y.

Now, build open sentences.
x + y = 12
x - y = 4

Now, which method will we use to solve?  I choose Elimination by Addition.

Add the equations together.
2x = 16

Divide both sides by 2
x = 8

Substitute
8 + y = 12

Subtract 8 from both sides.
y = 4

Check

The sum of the two numbers is 12 (8 + 4).

The difference of the two numbers is 4 (8 - 4).

The first number is 8.
The second number is 4.