Given, point C and D are on the line segment joining A(-4, 7) and B(5, 13) such that AC = CD = DB

So, the line is divided into 3 equals parts as shown in the given figure:

Now, point C divides the line in 1 : 2 as shown in the figure:

Now, coordinate of C = {(1 * 5 + 2 * (-4))/(1 + 2), (1 * 13 + 2 * 7)/(1 + 2)}

                               = {(5 - 8)/3, (13 + 14)/3} 

                               = (-3/3, 27/3)

                               = (-1, 9)

Again, point D divides the line in 2 : 1 as shown in the figure:

Now, coordinate of C = {(2 * 5 + 1 * (-4))/(1 + 2), (2 * 13 + 1 * 7)/(1 + 2)}

                               = {(10 - 4)/3, (26 + 7)/3} 

                               = (6/3, 33/3)

                               = (2, 11)

So, the coordinate of C is (-1, 9) and coordinate of D is (2, 11)