Geometry

Question Analysis:
This coordinate geometry problem tests understanding of parallelogram properties in the Cartesian plane, specifically how to find a missing vertex when three points are known.

Key Concept Explanation:
In a parallelogram:
1. The midpoints of the diagonals coincide (they bisect each other)
2. For vertices A, B, C, D in order: midpoint of AC = midpoint of BD
3. This property allows us to set up equations to find missing coordinates

Step-by-Step Solution:
1. Calculate midpoint of AC:
  - A(1, 2), C(5, 6)
  - Midpoint = ((1 + 5)/2, (2 + 6)/2) = (3, 4)
2. Let D = (x, y). Midpoint of BD must be (3, 4):
  - B(4, 3), D(x, y)
  - ((4 + x)/2, (3 + y)/2) = (3, 4)
3. Solve for x and y:
  - (4 + x)/2 = 3 → x = 2
  - (3 +...