Abstract Teaching mathematics is typically designed to develop the skills of learners, including problem-solving skills. Practice requires teachers to use effective teaching processes to bring the necessary mathematical skills back to the students. In many teaching models, attention must be paid to the lesson study model, which originated in Japan, because it has many educational meanings. This research aims to deploy a teaching process that promotes students' problem-solving skills to solve two mathematical problems in writing straight-line equations. The study used the model of a lesson study to do this. Two classes, including 74 students, have been selected for the control and experimental groups. For the first time, a lesson plan was taught in the control class; then it was thoughtfully adapted and taught for the second time in the experimental class. The data was based on the students' worksheets in two classes and qualitatively analysed to show progress in the teaching process. The findings indicated that the teaching process of the teacher's straight-line equations has led to a positive change in the ability of students to solve problems in the experimental class. These results also revealed more about the effectiveness of a lesson study model. Besides, some implications related to the professional development of teachers in teaching experience have been drawn up, such as teaching practice, competence, assessment skills and the lesson study team.