A plane contains the vectors $\mathbf{A}$ and $\mathbf{B}$. Determine the unit vector normal to the plane when $\mathbf{A}$ and $\mathbf{B}$ are equal to, respectively, $(a) 7 \mathbf{i}+8 \mathbf{j}$ $-2 \mathbf{k}$ and $9 \mathbf{i}-4 \mathbf{j}-5 \mathbf{k},(b) 6 \mathbf{i}-3 \mathbf{j}+9 \mathbf{k}$ and $-5 \mathbf{i}+4 \mathbf{j}-3 \mathbf{k}$