1. Subtraction is the inverse of addition. An addition formula can write two subtraction formulas.

And-one addend = another addend,

Appendix+Appendix = Sum

()+9=37

37-9=28

28+6=34

Second, addend+addend = sum

One addend remains the same, the other addend adds 3, and the sum also increases by 3 accordingly.

9-6=3

37-3=34

The correct result is 34.