This chapter presents the design outcome of the dissertation: a learning environment with an interactive instructional video developed iteratively across three design research cycles. The environment implements a two-phase problem-solving-before-instruction approach in which students first explore the content independently and then, in a knowledge organization phase, work with the interactive video and follow-up tasks to consolidate their ideas. Although the video is used after initial explorations, targeted tasks and scaffolds before, during, and after the video remain essential for enabling lower-achieving students to construct and consolidate challenging meanings of variables and expressions. The chapter elaborates on how generic mid-level design principles and features are tailored to the algebraic content of generalizing using expressions with letter variables.