We can add and subtract to find missing numbers. Look at this problem.

+ | 2 | = | 5 |

We can find the missing number like this.

5 | - | 2 | = | 3 |

Start with the answer, 5.

Subtract 2.

5 - 2 = 3.

The missing number is 3.

Now check.

3 | + | 2 | = | 5 |