The points 1 + i, 4 + 2i and 6 + 5i are three corners A, B and C of a parallelogram ABCD, in that…

On the Argand plane, $a + ib$ is the point $(a, b)$. The modulus $|z|$ is the distance from the origin, and $\bar z$ is the mirror image in the real axis. The distance between $z_1$ and $z_2$ is $|z_1 - z_2|$, and their midpoint is $\dfrac{z_1 + z_2}{2}$.