100p for an answer on a test

Let's first retrace her steps

using PEMDAS we know that we first have to do the operations in parenthesis then multiply before we add or subtract so we have

so far so good now lets check the other side


from this we can determine that all of her math is correct. However, to say that two expressions are equivalent you have to substitude the same number on both expressions and get the same answer. Since she did not do this, amd used x=1 on one expression and used x=-1 on th other her answer is incorrect.