Problems with mathematical concepts and algorithms are increasingly being demanded from students to better understand mathematics lessons. This study aims to describe the ability of students to understand mathematical concepts and algorithms as well as to analyze the difficulties experienced by students in understanding concepts and algorithms in mathematics learning after the learning process is carried out with the application of realistic mathematical education (RME) models assisted by

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... ator software. The subjects of the study were students of class X Mia 2 SMA Private ASSISI Siantar, totaling 31 students. The research instrument is a test of the ability to understand concepts, mathematical algorithms and interview guidelines. Data analysis was carried out using the Miles and Huberman model. The results showed: (1) The students' ability to understand mathematical concepts on the material of a system of linear equations. Three variables in the Assisi Siantar High School students in class x mia 2 belong to the Medium category, this can be seen from the results of the average score of each indicator, worth in 59.43. (2) Ability of Mathematical Algorithm students of SMA Assisi Siantar class x mia 2 on the material of linear equation system. Three variables are classified at the moderate level of ability, this can be seen from the results of the score on each indicator that is still in the average category of 55.2. (3) The difficulties experienced by students in completing the ability test for understanding mathematical concepts and algorithms in learning mathematics with the Realistic Mathematical Education learning model with the help of Algebrator software have difficulty understanding concepts, difficulties in applying principles, and also difficulties in implementing operations.