Algrebra
posted by Anonymous .
The points (1,5), (4,5), and (1,0) represent three corners of a rectangle; find the coordinates of the fourth poin

Those three coordinates should tell you that the rectangle has sides parallel to the x and y axes. This makes the problem simple. The first two points are the ends of a horizontal line at y =5, with length 3.
The first and third points are the ends of a vertical line at x=0 with length 5.
A point at (4,0) will complete the rectangle with a side at x=4 of length 5, and a side at y=0 with length 3.
If you can't follow my argument, try plotting the points on a graph.