If vectors $\overrightarrow{a}=\widehat{i}+2\widehat{j}-\widehat{k},\overrightarrow{b}=2\widehat{i}-\widehat{j}+\widehat{k}and\overrightarrow{c}=\lambda \widehat{i}+\widehat{j}+2\widehat{k}$ are coplanar, then find the value of $\left(\lambda -4\right)$

vectors $\overrightarrow{a}=\widehat{i}+2\widehat{j}-\widehat{k},\overrightarrow{b}=2\widehat{i}-\widehat{j}+\widehat{k}and\overrightarrow{c}=\lambda \widehat{i}+\widehat{j}+2\widehat{k}$ are coplanar

$\Rightarrow \left|\begin{array}{ccc}1& 2& -1\\ 2& -1& 1\\ \lambda & 1& 2\end{array}\right|=0$

$or \lambda -3+2(-5)=0$

$or \lambda = 13$