The answer is 21
By comparing the 4 vertices you can determine the rectangle x-length and y-length. The coordinate for the vertice is:
1,+4i
-2, +4i
-2, -3i
1, -3i
From the coordinate, you can see there are 2 x coordinate(1 and -2) and 2 y coordinate(4i and -3i) that used.
The x-length would be: 1 - (-2)= 3
The y-length would be: 4i- (-3i)= 7i
Then the area of the rectangle would be:
area= x-length * y-length= 3*7=21