The angular momentum of a particle is defined by .
The contribution of each mass to the angular momentum is of the form L=mrv since in each case the velocity is perpendicular to the displacement from the origin. In each case the vector points perpendicular to the plane, but in opposite directions: the particle on the x-axis gives a contribution out of the page; the particle on the y-axis gives a contribution into the page.
The total angular momentum is
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