Three vertices of parallelogram ABCD are A(-5, -1), B(3, -1) and C(1, -6). Use graphical method to find the co-ordinates of fourth vertex D.

Plot the points A(-5, -1), B(3, -1) and C(1, -6) on the graph paper. Join point A with B and B with C.

From the graph, it is clear that the horizontal distance between the points A (-5, -1) and B (3, -1) is 8 units and the vertical distance between the points B (3, -1) and C (1, -6) is 5 units. Therefore, the vertical distance between the points A (-5, -1) and D must be 5 units and the horizontal distance between the points C (1, -6) and D must be 8 units.

Now, complete the parallelogram ABCD and read the coordinates of point D. As shown on the graph, D = (-7, -6).

Hence, the co-ordinates of fourth vertex D are (-7, -6).