ABCD is a parallelogram, where A is (4,2), B is (-6,1) and D is (-3,-4). Find the coordinates of C.

My attempt:

First I'll find the vector then solve for its magnitude:

Now I'll find the magnitude:

Unfortunately, this is the farthest I can get. I tried isolating for but didn't work out as I only got 1 value and not two. I don't have the slightest clue on how to solve this problem (never saw it in my work or problems) and it threw me off completely. Thanks in advance.