It is an opportunity to help the student become aware of the fact that the information needed is frequently in the problem statement. It is a systematic approach to gathering the essential information needed to effect solution of the problem. In problems encountered in early grades this can be done in the head by some students. These will resist recording the steps, or even being consciously aware of the steps, and lose the opportunity to develop skills of value in later work. The majority of the students are denied the opportunity to use mathematics as a language and develop mathematical skills of permanent value if these steps are glossed over.
Substitution is a process extensively used in mathematics.
Follow the process here. The first equation is a response to what is asked for. The next two equations respond to requests by the starting equation. The responses are then substituted in the starting equation. Simplification of the result of substitution provides the final answer.