(a²-3ab+b²)+(-a²+3ab+b²)

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

First, let's simplify the terms within each set of parentheses: (a²-3ab+b²) becomes a²-3ab+b² (-a²+3ab+b²) remains as -a²+3ab+b²

Now, let's combine like terms by adding the corresponding coefficients of like terms: a² + (-a²) = 0 (the a² terms cancel each other out) -3ab + 3ab = 0 (the -3ab and 3ab terms cancel each other out) b² + b² = 2b²

Therefore, the simplified expression is 0 + 2b², which can be further simplified to just 2b².