She is applying the anti-commutative law properties of subtraction; a-b = - (b-a)<p>Also note that you can do this with division. A lot of people are taught that anti-commutativity is only applicable to subtraction but it also applies to any operator or function.<p>a op b = Identity(op) op ( b op a )<p>Obviously that assume that a op b is defined and b op a is defined and that an identity exists under the operator op.<p>Same goes for functions,<p>f(a,b) = f(Identity(f()),f(b,a))
It's interesting that the poster knows "subtrahend" and "minuend" and fully explains the algorithm, but can't solve the pre-algebra problem that shows that it is accurate.