This is true if the vector field is conservative. A vector field $F(x, y) = P(x, y)i + Q(x, y)j$ is conservative if and only if $\frac{\partial P}{\partial y} = \frac{\partial Q}{\partial x}$.
In our case, $P(x, y) = 3y$ and $Q(x, y) = 3x$. So, we need to check
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