This study aimed to assess Form Four students' algebraic solving ability using linear equations across four content domains: linear patterns, direct variation, concepts of functions, and arithmetic sequences. Students took a pencil-and-paper test consisting of items assessing different levels of algebraic understanding based on the SOLO model. Clinical interviews were also conducted. Results showed that 62% of students had less than 50% probability of success at the relational level. Most students were at the unistructural or multistructural levels. High-ability students could identify patterns and relationships between variables more easily than low-ability students. The study provided evidence that the SOLO model is useful for assessing algebraic solving ability in upper secondary school.