(a²-3ab+b²)+(-a²+3ab+b²)

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

First, let's remove the parentheses: a² - 3ab + b² - a² + 3ab + b²

Now, let's combine like terms: (a² - a²) + (-3ab + 3ab) + (b² + b²)

The terms with a² and -a² will cancel each other out: 0 + (-3ab + 3ab) + (b² + b²)

The terms with -3ab and +3ab will cancel each other out as well: 0 + 0 + (b² + b²)

Finally, we can simplify the remaining terms: 0 + 0 + 2b²

Therefore, the simplified expression is: 2b²