Aleksandrova’s books usually provide 4 variants for each test. Try solving Variant 1 with the answers open, then solve Variant 2 entirely on your own to test your knowledge.
Grade 8 Algebra Control Works (Aleksandrova): Solutions & Prep Guide
Контрольные работы по алгебре 8 класс - Инфоурок
Use the answers to verify your final result. If it doesn't match, work backward to find where your calculation went wrong.
Addition, subtraction, multiplication, and division. Quadratic Roots: Working with square roots and the equation