P. R. Salihah, S. Sahiruddin, and P. D. D. Degeng, “An Analysis of Text Readability in 9th Grade English Textbook Using FRE Formula on Readable Software”, diglossia. jurnal. kaji. ilm. kebahasaan. dan. kesusastraan, vol. 11, no. 1, pp. 24–33, Sep. 2019.