The two diagonals $AC$ and $BD$ of the quadrilateral $ABCD$ intersects at point $M$ and divides the quadrilateral into four triangles $(\triangle AMB, \triangle BMC, \triangle CMB \& \triangle DMA)$. If $\triangle AMB, \triangle BMC, \triangle CMD$ have areas of $1, 2, 3$ square unit respectively, find the area of $\triangle DMA$.