Answer
${\bf( 2i-j)}\times{\bf (i+2j)}=5{\bf k}$
Work Step by Step
${\bf u}\times{\bf v}=\left|\begin{array}{lll}
{\bf i} & {\bf j} & {\bf k}\\
u_{1} & u_{2} & u_{3}\\
v_{1} & v_{2} & v_{3}
\end{array}\right|$
---
${\bf( 2i-j)}\times{\bf (i+2j)}=\left|\begin{array}{rrr}
{\bf i} & {\bf j} & {\bf k}\\
2 & -1 & 0\\
1 & 2 & 0
\end{array}\right|=(0)-(0)+(4+1){\bf k}$
$=5{\bf k}$
