Research examining instruction in geometry and standardized tests suggests that students have difficulty grasping geometry concepts and developing problem solving skills. The purpose of this study was to examine the relationship between the use of inquiry-based strategies in a geometry class and achievement on the end of course test (EOCT) and to analyze qualitatively the implementation of inquiry-based instruction. Embedded in the theoretical framework of constructivism, inquiry-based instruction gives students skills to become independent learners. Addressing an issue in mathematics education, the primary research question focused on how to improve scores on a standardized geometry test. This mixed methods study utilized the t test to analyze the EOCT scores of 2 groups of geometry students in a Title I school. The results indicated that students taught using inquiry-based instruction scored higher on the EOCT. Lesson plans, field notes, observation notes and other artifacts were analyzed using categorical aggregation. The results indicated that the predominant instructional strategy in the implementation process was guided inquiry and that formal instruction included models of the inquiry process. Social change will be impacted by pointing to instructional strategies that will help students develop positive attitudes to problem solving through inquiry and increased understanding of the mathematical content. The development of critical thinking skills in problem solving will contribute to success in high school, in college and in the workplace.