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Answer:
Explanation:
(-)(-) = (+)
(-)(+) = (-)
(+)(-) = (-)
(+)(+) = (+)
-(-21) = (-)(-21) = (+)21 = 21
-12 - (-21) = -12 + 21 use commutative property: a + b = b + a
= 21 + (-12) = 21 - 12 = 9