The body of literature on a Writing-to-Learn (WTL) approach in math courses offers up a variety of assignment types from which to choose. However, few of these articles provide empirical evidence on the ways these writing assignments contribute to students' learning. This mixed-methods study, conducted at the University of New Mexico, a Hispanic-Serving Institution, examines the effect of WTL assignments on students' success in two "killer courses": a Survey of Math class for non-STEM majors and a Calculus I class for STEM majors. While the quantitative results did not prove statistically significant, the qualitative results suggest that high-impact assignments are those that ask students to focus on procedural knowledge, or analyzing the process, rather than simply solving for the right answer.