A force $F=2\hat{\mathbf{i}}+\hat{\mathbf{j}}-\hat{\mathbf{k}}$ acts at a point $A$, whose position vector is $2\hat{\mathbf{i}}-\hat{\mathbf{j}}$. The moment of $F$ about the origin is

Force $\left(F\right)=2\hat{\mathbf{i}}+\hat{\mathbf{j}}-\hat{\mathbf{k}}$ and its position vector $=2\hat{\mathbf{i}}-\hat{\mathbf{j}}$. 

We know that the position vector of a force about origin $\left(r\right)=(2\hat{\mathbf{i}}-\hat{\mathbf{j}})-(0\hat{\mathbf{i}}+0\hat{\mathbf{j}}+0\hat{\mathbf{k}})$ or $r=2\hat{\mathbf{i}}-\hat{\mathbf{j}}$ 

Therefore, moment of the force about origin
$=r\times F=\left|\begin{array}{ccc}\hat{\mathbf{i}}& \hat{\mathbf{j}}& \hat{\mathbf{k}}\\ 2& -1& 0\\ 2& 1& -1\end{array}\right|=\hat{\mathbf{i}}+2\hat{\mathbf{j}}+4\hat{\mathbf{k}}$