Answer
$2i-j+k$
see the attached image.
Work Step by Step
$a\times b=(1-2k) \times (j+k)= \begin{vmatrix} i&j&k \\ 1& 0&-2\\0&1&1\end{vmatrix}$
Expand along the first row:
$a\times b=\ i\begin {vmatrix} 0&-2 \\ 1&1\end{vmatrix}-\ j \begin {vmatrix} 1&-2 \\ 0&1\end{vmatrix}+\ k\begin {vmatrix} 1&0 \\ 0&1\end{vmatrix}$
$a \times b=2i-j+k$
The three vectors are depicted as below:
