This article introduces activity theory and how it can be employed to instruct the topic of straight-line equations in a plane - Geometry 10. Using the activity theory approach, we studied and developed a teaching process. The procedure is divided into three stages: Phase 1: Motivation and goal orientation, in which teachers present situations to attract students to the lesson; Phase 2: Knowledge formation, in which students engage in a variety of learning activities to build the knowledge they need to learn; Phase 3: Practice and consolidation, in which many exercises are assigned to students to solve in order to consolidate their knowledge and assist teachers in detecting and correcting students' misconceptions. To examine the effectiveness of applying the proposed three-phase model, we used a two-group pretest-posttest experimental model to determine whether or not teaching with the activity theory approach is more effective than the traditional teaching method by testing four research hypotheses. The experimental teaching took place in the Mo Cay district of Ben Tre province, Vietnam. Both the experimental and control classes began with the same level of mathematics, which was then tested using inferential statistics. After completing the pedagogical experiment, we discovered that students in the experimental class who were taught using activity theory achieved better learning outcomes than students in the control class, who were taught using the traditional teaching method; in the experimental class, the number of weak students decreased in comparison to the original; however, the number of good students did not increase. This is an issue that requires further studies to find ways to influence a wide range of students with different levels of mathematics so that the effectiveness of teaching according to the activity theory approach is improved.