My homework question says given:
$\left|\begin{matrix}a & b& c\\ d& e& f\\ g& h& i\end{matrix}\right| = -6$
evaluate the determinant
$\left|\begin{matrix}a+d & b+e& c+f\\ -d& -e& -f\\ g& h& i\end{matrix}\right|$
It says I have to do this by row reduction. I know how to row reduce but I don't know where to start for this question. I just can't grasp how the two matrixes/determinants can be used together to solve the question.
Any help would be greatly appricated