My students used these centers to prepare for our systems of equations exam. The three centers practiced word problems, mixed solving skills (graphing, substitution, elimination), and a justification exercise in which students had to answer why certain problems are solved using substitution vs. elimination.
Justification is a skill I focus on to make sure students know how to make their own solving decisions. I find that without justification practice, on assessments students struggle when substitution and elimination are mixed together.
Also included is an exit ticket to assess student mastery on the skills covered.