[math]a=\left(B-A\right)=\left(\binom{7}{5}-\binom{4}{2}\right)=\binom{3}{3}[/math][br][math]b=\left(C-B\right)=\left(\binom{6}{y}-\binom{7}{5}\right)=\binom{-1}{y-5}[/math][br][math]a\cdot b=0\Longrightarrow\binom{3}{3}\cdot\binom{-1}{y-5}=0\Rightarrow3\cdot\left(-1\right)+3\cdot\left(y-5\right)=0\Rightarrow-3+3y-15=0\Rightarrow3y=18\Rightarrow y=6\Rightarrow C=\left(1\left|6\right|\right)[/math][math]D=A+b=\binom{4}{2}+\binom{-1}{6-5}=\binom{4}{3}+\binom{-1}{1}=\binom{3}{4}[/math]