The error in Brianna's thinking is that she only tested the expressions when x is equal to 0. However, to determine if the expressions are equivalent, we need to test them for all possible values of x.
Expression A: 9x - 3x - 4 = 6x - 4
Expression B: 12x - 4
Expression C: 5x + x - 4 = 6x - 4
Expressions A and C are equivalent because they both simplify to 6x - 4. Expression B is not equivalent to either of the other expressions because it cannot be simplified any further.
Hope this helps