Homework Help: Avg Velocity and Avg Speed

doing this problem 400km East in .75 hrs from a to b and 960 km south in 1.5 hrs from b to c.
Ang Velocity is /Delta x divided by /delta t correct?
and avg speed is just adding the distance's and times and diving by the time right?
Well for the avg velocity for this problem its saying its wrong any help?

 

found displacement by using a right-hand coordinate system. and did solved by doing a^2 + b^2 = c^2. got the displacement to be 1040 km for the total trip. (Checked it its right)
Did velocity avg by 960-400/1.5-.75 = 747 and it says its wrong....

 

well it said change in x over change in t
960 and 400 was the two distances, how to apply the displacement? do i subtract from that?!

 

but what does my schaums outline says change in x over change in timee, im confused :(
is it displacement over change in time or total time?

also isnt displacement delta x?