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The real part of complex number z= 2a+i\left ( 7-a \right ) when z becomes purely real will be \left ( a\: \epsilon\: R \right )

Let \underset{u}{\rightarrow} be a vector coplanar with the vectors \underset{a}{\rightarrow}= 2\hat{i}+3\hat{j}-\hat{k}  and \underset{b}{\rightarrow}= \hat{j}+\hat{k} . If \underset{u}{\rightarrow} is perpendicular to \underset{a}{\rightarrow} and \underset{u}{\rightarrow}\underset{b}{\rightarrow}= 24, then \left | \vec{u} \right |^{2} is equal to :

Option 1)  84

Option 2)  336

Option 3)  315

Option 4)  256

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