In this ULF-project, researchers together with active teachers explore how it is possible to establish a link between teaching related questions posed by teachers and the ongoing research efforts, within the framework of a professional arena. How can a sustainable collaboration between academia and teaching practice take place? To examine this, we focus on a specific subject content, the equation of linear functions (y=kx+m). In our practice, this has been a challenge for teaching. To specify the object of learning, variation theory along with Cultural Historical Activity Theory inspired by Davydov (Tambour, 2019) has been used. The overarching question “What does the knower know?” act as a mediating tool to discern and develop reasoning (Carlgren, 2023; Kullberg et al., 2017). This design takes teachers’ point of view as a starting point, rather than researchers' preconceived notions of what constitutes an interesting subject of study in schools. The arena could be a

tool to strengthen the teacher's professional development through a locally designed competence development initiative in an educational setting characterized by changes.