Diagonals bisect at M $(-1, -2)$.
Sum of coordinates of A + C = $(1-3) + (2-6) = -2 -4 = -6$.
In a rhombus (parallelogram), $x_A+x_C = x_B+x_D$ and $y_A+y_C = y_B+y_D$.
$\alpha + \gamma = -2$.
$\beta + \delta = -4$.
Sum = $-6$. Magnitude is 6.
No need to find coordinates explicitly.