(a²-3ab+b²)+(-a²+3ab+b²)

To simplify the expression, you can combine like terms.

(a² - 3ab + b²) + (-a² + 3ab + b²)

Now, let's group the terms together:

(a² + (-a²)) + (-3ab + 3ab) + (b² + b²)

The terms (a² + (-a²)) and (-3ab + 3ab) simplify to 0 since they are additive inverses of each other. Thus, we are left with:

0 + 0 + (b² + b²)

Simplifying further:

0 + 0 + 2b²

Finally, combining like terms:

2b²

Therefore, the simplified expression is 2b².