T-algebra is an intelligent learning environment for step-by-step solving of algebra problems in four areas of school mathematics, including linear equations. The knowledge the system has of its subject domain is one of key places for intelligence in learning systems. T-algebra domain expert module follows the 'glass-box' principle and can produce step-by-step solutions similar to pencil-and-paper solutions. In addition to solving the problems, the domain expert module in T-algebra can also check the student's solution steps and answers, give advice, etc. This paper will describe a domain expert module built in T-algebra intelligent environment and its applications using the domain of linear equations as an example.