Question

Brianna's teacher asks her which of these three expressions are equivalent to each other.

Answer 1

Brianna's thinking is incorrect.
Expessions A, and C. are equivalent.
For C, you add 5x and x together (because they have the same variable) to get 6x - 4.
For A, you subtract 9x and 3x (because they have the same variable) to get 6x- 4.

Answer 2

Brianna is correct about the situation when x=0.  But you can't say that
expressions are really equivalent unless they have the same value for
EVERY 'x', no matter what it is.

Expressions 'A' and 'C' will do that.  But Expression 'B' won't, unless x=0.