In science and engineering courses, students are often presented a situation for which they are asked to identify the relevant principles and to instantiate them as a set of equations. For an ITS to determine the correctness and relevance of the student's answer and generate effective feedback, it must map the student variables and equations onto the physical properties and concepts that are relevant to the situation. The space of possible mappings of variables and equations is extremely large. Domain independent techniques by themselves are unable to overcome the complexity hurdles. This paper describes how an ITS can use constraint propagation and algebraic techniques combined with domain and problem-specific knowledge to solve the mapping problem with systems of algebraic equations. The techniques described in this paper have been implemented in the PHYSICS TUTOR tutoring system and evaluated on three data sets that contain submissions from students in several introductory Physics courses.