“…In [9], Jupri, Drijvers and Van den Heuvel-Panhuizen identified five main categories of difficulties in algebra learning. First, the difficulties in using arithmetical operations and properties include difficulties in adding like terms (e.g., [10]; [11]); in using commutative, associative, inverse, and distributive properties; and in using priority rules of arithmetical operations (e.g., [12]; [13]). Second, the difficulties in understanding the notion of variable concern understanding it as an unknown, a generalized number, a placeholder, or a varying quantity ( [10]; [12]).…”