Stop the "Missing Side" frustration and turn your students into Geometry Detectives! This no-prep, high-engagement Google Slides lesson is designed to take the mystery out of finding unknown side lengths. Directly aligned with CCSS 3.MD.D.8, this resource moves students from basic addition to the algebraic thinking required for 3rd-grade perimeter standards. The Problem: Students often know how to add sides to find perimeter, but they get stuck when a side is labeled "S" or "X." They don't alw