(a²-3ab+b²)+(-a²+3ab+b²)

To simplify the expression (a²-3ab+b²)+(-a²+3ab+b²), we can first remove the parenthesis and then combine like terms.

(a²-3ab+b²) + (-a²+3ab+b²)

Now, let's remove the parenthesis.

a² - 3ab + b² - a² + 3ab + b²

Next, let's combine like terms.

(a² - a²) + (-3ab + 3ab) + (b² + b²)

Since a² - a² equals 0 and -3ab + 3ab equals 0, we can simplify further.

0 + 0 + 2b²

Finally, we have 2b² as the simplified expression.