(a²-3ab+b²)+(-a²+3ab+b²)

To simplify the expression (a²-3ab+b²)+(-a²+3ab+b²), we can combine like terms:

(a²-3ab+b²)+(-a²+3ab+b²) = a² - 3ab + b² - a² + 3ab + b² = (a² - a²) + (-3ab + 3ab) + (b² + b²) = 0 + 0 + 2b² = 2b²

Therefore, the simplified expression is 2b².