In Step 6, the equation is ...
-12 = x - 36
In step 7, the equation is ...
-12 +36 = x -36 +36
Clearly, to get to step 7, 36 has been added to both sides of the equation.
Adding 36 to both sides of the equation is justified by ...
... C) Addition Property of Equality
_____
<em>More about the addition property of equality</em>
The addition property of equality says you can add the same value to both sides of an equation without changing the values of the variables that make the equation true. Here, x=24 makes the equation of step 6 true, and x=24 makes the equation of step 7 true.