Hi there! Hopefully this helps!
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Answer:
![\boxed{21}](https://img.qammunity.org/2021/formulas/mathematics/high-school/pgyrjgh0oqpyzftqf52j1fzujyqunufsc8.png)
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When you multiply two negatives (-, -,), you are gonna get a positive(+)
[Like making a cross out of two lines].
![-3(-7) = +3(+7)](https://img.qammunity.org/2021/formulas/mathematics/high-school/cuc8gtupna5dwrtespzc2q406j748xbu5z.png)
Since 3(7) normally equals 21, that would be the answer for this case.