I am actually really stuck on this...

I found three different possible values of  d  using this graph:

https://www.desmos.com/calculator/cwbrobjoav

You can turn on the different parallelograms by clicking the circle next to the name.

Here's a picture with all of them turned on:

Let $\large{a = 4 + 3i}$, $\large{b = 1 -2i}$, and $\large{c = 8 - 5i}$.  
The complex number d is such that a, b, c, and d form the vertices of a parallelogram,
when plotted in the complex plane. Enter all possible values of d, separated by commas.

$\begin{array}{|rclclcl|} \hline \mathbf{d} &=& -a+b+c &=& -(4+3i)+ (1 -2i) + (8 - 5i) &=& \mathbf{5-10i} \\\\ \mathbf{d} &=& a-b+c &=& (4+3i)- (1 -2i) + (8 - 5i) &=& \mathbf{11+0i} \\\\ \mathbf{d} &=& a+b-c &=& (4+3i)+ (1 -2i) - (8 - 5i) &=& \mathbf{-3+6i} \\ \hline \end{array}$