Geometry
posted by debbie .
The endpoints of one diagonal of a rhombus are (0, 8) and (8, 4). If the coordinates of the 3rd vertex are (1, 0), what are the coordinates of the 4th vertex?
(7, 12)
(7, 8)
(8, 4)
(4, 12)

Simplest way, just count on the grid ...
you have 3 points, and you know that opposite sides must be parallel and equal
A quick calculation shows that each of the given sides is √65 , so far so good.
For the line from (1,0) to (0,8) we went one to the left, then 8 down
so from (8,4) go 1 left, then 8 down to get
(7, 11)
(check by taking the lengths of the new lines, I did,
they were both √65) 
/:) /xcfrff