Step 1: Understanding the Concept
The diagonals of a square are perpendicular to each other and bisect each other.
Step 2: Check the vertex
Put \((2,8)\) in \(x-y+3\): \(2-8+3=-3\ne 0\). So the vertex is not on the given diagonal and must lie on the other diagonal.
Step 3: Slope
The given diagonal has slope \(1\), so the other diagonal has slope \(-1\).
Step 4: Equation
\(y-8=-1(x-2)\Rightarrow x+y-10=0\)
Final Answer:
The other diagonal is x + y - 10 = 0.
\[ \boxed{\text{(A) }x+y-10=0} \]