Sarkum, S., B. A. Pramuka, and A. Suroso. “A Strategy Engagement in Marketing: A Reviews of the Literature”. International Journal of Sciences: Basic and Applied Research (IJSBAR), vol. 33, no. 2, May 2017, pp. 120-7, https://www.gssrr.org/index.php/JournalOfBasicAndApplied/article/view/6899.