You have 2 points with the same x value of 2 and 2 points with the same y value of 4.
You now need 2 points with the same x value of 4 ( you are given 1) and 2 y values of 1 ( you are given 1.
The missing point would need to be (4,1)
The width would be x2-x1 = 4-2 = 2 units
The length would be y2-y1 = 4-1 = 3 units
Area = 3x 2 = 6 square units