Explanation:
A Given,
\(\mathrm{A}(2,4,-1), \mathrm{B}(3,6,-1)\)
\(C(4,5,1)\) and let \(D(x, y, z)\)
Diagonals of parallelogram bisect each other then, Midpoint of \(\mathrm{AC}=\) midpoint of \(\mathrm{BD}\)
\(\Rightarrow\left(\frac{2+4}{2}, \frac{4+5}{2}, \frac{-1+1}{2}\right)=\left(\frac{3+\mathrm{x}}{2}, \frac{6+\mathrm{y}}{2}, \frac{-1+\mathrm{z}}{2}\right)\)
\(\| \Rightarrow\left(3, \frac{9}{2}, 0\right)=\left(\frac{3+x}{2}, \frac{6+y}{2}, \frac{-1+\mathrm{z}}{2}\right)\)
\(\Rightarrow \mathrm{x}=3, \mathrm{y}=3, \mathrm{z}=1\)
\(\Rightarrow \mathrm{D}=(3,3,1)\)