Look at the drawing given in the figure which has been drawn with ink of uniform line-thickness. The mass of ink used to draw each of the two inner circles, and each of the two line segments is m . The mass of the ink used to draw the outer circle is 6 m . The coordinates of the centres of the different parts are: outer circle $(0,0)$ left inner circle $(-\mathrm{a}, \mathrm{a})$, right inner circle $(\mathrm{a}, \mathrm{a})$, vertical line $(0,0)$ and horizontal line $(0,-a)$. The $y$-coordinate of the centre of mass of the ink in this drawing is