Question

How can 9+3 =12 help you solve 12-9?

Answer 1

9+3=12, 12-9=3, since those three numbers are in the first equation.

Answer 2

Well you know that 9+3=12 so you can reverse it and do 12-9=3 .Because 3 is the only one left in the equation of 9+3=12.