[ \begincases 3x - 2y = 7\ 5x + 4y = -1 \endcases ]
The workbook is organized into modular units, each focusing on specific communicative competencies. The answer key validates these modules by providing solutions for vocabulary building, grammar application, and reading comprehension. Intermediate (CEFR B1).
(x = 1,\qquad y = -1)
Research in second language acquisition shows that immediate feedback significantly improves retention. Instead of waiting for a teacher to grade your work (sometimes days later), you can check each exercise as you complete it.