(4A-4)

①  Since A is (0,-2) and B is (-2,-2), distance 0A =2 and distance AB = 2.

Therefore, using Pythagoras' theorem, distance 0B = (22 +  22 =  22.

②  Since the equation of the plane is z = -x - 2y, the z coordinate at point C is given by z = 2 + 4 = 6.

Therefore distance BC = 6. 

③ The gradient of the yellow line is the ratio of the distances, BC/B0  =  6 / 22   =  3/2.  Since the change in z is an increase but the changes in x and y are negative, this is a negative gradient:

So the slope of the plane in the direction x = y is equal to -3/2.