Find the other two vertices of that square whose opposite vertices are (1,1) and (1,8).

Answer 1

First find the length of the diagonal from (1,1) to (1,8)
you can do this in your head , and it is 7, and half of it is 3.5
so the centre of your square is (1, 4.5)

notice since the first diagonal is vertical, the second one must be horizontal.

The diagonals of the square bisect each other, and the diagonals are equal, so just add 3.5 to the 1 to get (4.5, 4.5) as one of the other points, then subtract 3.5 from 1 to get
(-2.5, 4.5) for the last point.