The correct answer is 4a^2 + 3ab
In order to find this, we can write the equation out as described and then follow the order of operations.
3a^2 - 2ab + 3b^2 - (-a^2 - 5ab + 3b^2) ----> Distribute the negative
3a^2 - 2ab + 3b^2 + a^2 + 5ab - 3b^2 ----> Combine like terms
4a^2 + 3ab