Let D(x, y, z) be the fourth vertex of parallelogram ABCD. We know that diagonals of a parallelogram bisect each other. So the mid point of AC and BD coincide.
∴ Coordinates of midpoint of AC are
(

3−1

2

,

−1+1

2

,

2+2

2

) = (1 , 0 , 2)
Also coordinates of mid point of BD are
(

x+1

2

,

y+2

2

,

z−4

2

)
∴

x+1

2

= 1 ⇒ x + 1 = 2 ⇒ x = 1

y+2

2

= 0 ⇒ y + 2 = 0 ⇒ y = - 2

z−4

2

= 2 ⇒ z - 4 = 4 ⇒ z = 8
Thus the coordinates of point D are (1, –2, 8)