There vertices of a rectangle are (-1,0) (6,0), and (6,3) . what are the coordinates of the forth corner point of the rectangle?

The point will be at (-1, 3). Use the x value that has only been used once, -1, and the y value that has only been used once. (6,0) and (6, 3) are on the same side vertically of the rectangle because their x values are the same. Because 6 > -1, you know that they make up the right side of the rectangle.  (-1,0) and (6, 0) both have the same y values, so they are on the same side horizontally, and are the bottom of the rectangle because 0<3. This all means the fourth point is the rectangles's top left corner. Because it is a rectangle, its sides are parallel, so you know that it will have the same x value as the left bottom point (-1,0) and the same y value as the right top point, (6, 3).