Evaluating closed path integral $\oint_C(aydx +2xydy)$ without greens theorem

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The path is a rectangle in the x-y plane with vertices $(0,0), (a,0), (0,b),(a,b)$. How would I do this without greens theorem? I tried to parametrise it but that didn't work as I don't have functions of just $x$ and $y$.