G(1) is equal to -3 because when x is 1, y is -3.
Now, find G(-3). So go to the x-coordinate of -3, and see what the y-coordinate is. In this case, the y-coordinate is 5.
Now you are left with G(G(G(5))).
Now here, when the x-coordinate is 5, the y-coordinate is also 5. Since you must take the y-coordinate from G(x) = y, you are left with G(G(5))
I believe you know what this means.