This case study examines a sixth-grade teacher and her students in an urban school district in Alaska, engaging in an activity from a module that is part of the Math in a Cultural Context (MCC) series. By analyzing the module, the teacher's practice, classroom discourse, and students' work, the case shows that the teacher and the MCC module supported students in developing substantive reasoning and understanding about the mathematical relationship between constant perimeter and varying area in rectangles. Comparison of students' scores on pre- and post-tests show that the class as a whole outperformed the control group. Moreover, Alaska Native students, comprising slightly over one-fourth of the class, outperformed the control group, had gains in achievement commensurate with the entire class, and outperformed their Alaska Native peers in the control group by a wide margin. The case shows that the MCC module and the teacher's practice support improved mathematics achievement through interwoven connections between content, pedagogy, and culture.