Sari, P., M. N. Hajizah, and S. Purwanto. “The Neutralization on an Empty Number Line Model for Integer Additions and Subtractions: Is It Helpful?”. Journal on Mathematics Education, vol. 11, no. 1, Jan. 2020, pp. 1-16, http://jme.ejournal.unsri.ac.id/index.php/jme/article/view/3677.