To answer the question, first you need to work out the distance between the points, which was 9 units.
Then you say 9* what = 90
the 'what' is the other dimension of the rectangle. 9*10=90
so the other side must be 10 units long. (10 rows of 9 little squares)
with that you can work out where the points C and D go
You already did that :))