Answer
$ [\ 12\ \ -9\ \ \ 3\ ]$
Work Step by Step
1. Check if the matrices have the same dimension. If they don't, the sum/difference is not defined.
2. The sum/difference will be a matrix with the same dimension.
3. Position by position, add/subtract the corresponding entries.
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$[\ 10\ -4\ \ \ 6\ ]-[-2\ \ 5\ \ 3\ ]=$
... dimensions match, continue
$=[\ 10-(-2)\ \ -4-5\ \ \ 6-3\ ]$
$=[\ 12\ \ -9\ \ \ 3\ ]$
