Must-know identities
(a+b)² = a² + 2ab + b², (a-b)² = a² - 2ab + b², and a² - b² = (a-b)(a+b) appear constantly.
Before expanding everything, check whether an identity can collapse the expression in one step.
Equation hygiene
Always do the same operation to both sides, isolate the variable last, and substitute your answer back once. That final check catches sign errors that cost full marks.