Explain how to use diagrams for 13 - 6 by the subtraction method.

Imagine drawing 13 small squares or circles.

$$\text{Diagram 1: } \square \square \square \square \square \square \square \square \square \square \square \square \square$$

(This represents 13 blocks.)

From the 13 blocks, cross out or remove 6 of them.

$$\text{Diagram 2: } \cancel{\square} \cancel{\square} \cancel{\square} \cancel{\square} \cancel{\square} \cancel{\square} \square \square \square \square \square \square \square$$

(6 blocks are crossed out.)

Count the blocks that are not crossed out.

$$\text{Diagram 3: } \text{The remaining blocks are: } \square \square \square \square \square \square \square$$

There are 7 blocks left.

$$13 - 6 = 7$$

The final answer is $\text{7}$.