After you guess what kind of quadrilateral is created by the four points, think about what would need to be true in order to show it.

Think:

Parallel lines (same slope)

Perpendicular lines (opposite reciprocal slopes [a/b and -b/a])

Equal lengths (use distance formula)

Slope formula: (y2-y1)/(x2-x1)

Distance: Create a right triangle, find the lengths of the legs (count the units). Once you know those two sides, you can use the pythagorean theorem to find the missing length. (a^2+b^2=c^2)