Let's divide the 2nd equation by 3: We get -3x - 3y = -10.
If we add this result to the 1st equation, we get 0x + 0y = 0, which has infinitelyl many solutions.
Alternatively, mult. all three terms of 3x + 3y = 10 by 3, to obtain 9x + 9y = 30.
Add this result to the 2nd equation. We get 0x + 0y = 0, which (again) is always true for all x and y values. This system does not have a unique solution.