\begin{pmatrix} 15 \\ 12 \\ -21 \end{pmatrix}
<|im_start|>system
You are a helpful assistant.<|im_end|>
<|im_start|>user
If $\mathbf{a} \times \mathbf{b} = \begin{pmatrix} 5 \\ 4 \\ -7 \end{pmatrix},$ then compute $\mathbf{a} \times (3 \mathbf{b}).$
Please reason step by step, and put your final answer within \boxed{{}}.<|im_end|>
<|im_start|>assistant

user
\begin{pmatrix} 15 \\ 12 \\ -21 \end{pmatrix}
If $\mathbf{a} \times \mathbf{b} = \begin{pmatrix} 5 \\ 4 \\ -7 \end{pmatrix},$ then compute $\mathbf{a} \times (3 \mathbf{b}).$
train