How do you write a system
What's a 'situation?' Possibly a a description, in words and numbers, describing some relationships.
If you get n equations in n variables from the algebraic interpretation of the given words, then you have a system of n equations (in n 'unknowns', or variables) that can be solved by algebraic methods.
I'll bet that (if you have a good algebra book) there are numerous examples of systems of equations. Later on, you'll encounter systems of inequalities, too.