This was designed for an 8th grade Pre-Algebra class. The background knowledge needed for this lesson includes ratios, proportions, cross-multiplication, and basic definitions of similar polygons and the notation surrounding them.
It walks students through the steps to find a missing side length by first giving them a pair of similar figures in which the calculation may done mentally and asks them to use that problem to derive the basic steps for finding a missing side.
I used it as homework to challenge students to draw their own conclusions about finding a missing side, then followed up with direct instruction the next day.
Both a PDF and Word file are included to edit as necessary.
The images are NOT my original work, but all other content is.