Dealing with unknowns for solving linear equations can be particularly challenging for students with MLD, but research findings suggest that the use of digital artifacts supports these students in constructing algebraic meanings. Using the Theory of Semiotic Mediation, we analyse a case study of a high school student with MLD, focusing on the evolution of signs that were developed during tutoring sessions with the use of Dynamic Digital Artifacts which embeds a balance model of linear equations. A combined use of quantitative and qualitative analytical tools leads us to describing the features of the student and researcher's semiotic activity that support or impede the development of mathematical signs for the notions of unknown and solution of a linear equation.