Answer:
Substitution
Step-by-step explanation:
In your premises, the state AB = BC is given to be true. In math, substitution is taking one value in an equation and "subbing it out for" one that we know has the same value. This means anywhere we see AB, we can substitute BC and the equation will still be true, because AB = BC.
In the example given, that changes AB + BC = AC to AB + AB = AC.
We substituted the AB with BC, because we know that they are of the same value.
