Let $a = 4 + 3i,$ $b = 1 -2i,$ and $c = 8 - 5i.$  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.
In the parallelogram, suppose that $d$ is opposite $a.$

[asy]
unitsize(0.4 cm);

pair A, B, C, D;

A = (4,3);
B = (1,-2);
C = (8,-5);
D = B + C - A;

draw(A--B--D--C--cycle);
draw(A--D,dashed);
draw(B--C,dashed);

dot("$a$", A, N);
dot("$b$", B, W);
dot("$c$", C, E);
dot("$d$", D, S);
dot((A + D)/2);
[/asy]

Recall that in any parallelogram, the midpoints of the diagonals coincide.  Hence,
\[\frac{a + d}{2} = \frac{b + c}{2},\]which leads to $d = b + c - a.$

The complex number $d$ can also be opposite $b$ or $c,$ which leads to the possible values $d = a + c - b$ and $d = a + b - c.$

[asy]
unitsize(0.4 cm);

pair A, B, C, D;

A = (4,3);
B = (1,-2);
C = (8,-5);

draw((B + C - A)--(A + C - B)--(A + B - C)--cycle,dashed);
draw(A--B--C--cycle,dashed);

dot("$a$", A, N);
dot("$b$", B, W);
dot("$c$", C, E);

dot("$b + c - a$", B + C - A, S);
dot("$a + c - b$", A + C - B, E);
dot("$a + b - c$", A + B - C, NW);
[/asy]

Thus, the possible values of $d$ are
\begin{align*}
b + c - a &= (1 - 2i) + (8 - 5i) - (4 + 3i) = \boxed{5 - 10i}, \\
a + c - b &= (4 + 3i) + (8 - 5i) - (1 - 2i) = \boxed{11}, \\
a + b - c &= (4 + 3i) + (1 - 2i) - (8 - 5i) = \boxed{-3 + 6i}.
\end{align*}