Hello, de1043!
Find a fourth point $\displaystyle D$ so that a parallelogram is formed
using the vertices $\displaystyle A(0,4),\; B(5,3),\; C(4,3)\text{ and }D$ in any order.
Plot your points and draw the parallelogram in the coordinate plane. There are three possible parallelograms.
Code:


          +           
 D
 o (1,2)
 * *
C *  * B
(4,3) o  o (5,3)
*  *
*  *
o
A(0,4)

Code:


          +           



C  B
(4,3) o * * * * * * * * o (5,3)
*  *
*  * D
o * * * * * * * * o
A(0,4) (9,4)

Code:


          +           



C  B
(4,3) o * * * * * * * * o (5,3)
*  *
D *  *
o * * * * * * * * o
(9,4) A(0,4)

I'll let you figure out how I located those points.