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-3x⁷ + 12x³ + 3x + 3

Explanation:
Before we begin, remember the following:
(-ve) * (-ve) = +ve
(-ve) * (+ve) = -ve
(+ve) * (+ve) = +ve
(+ve) * (-ve) = -ve

Now, for the given we have:
(12x³ + 7x + 3) - (3x⁷ + 4x)

First, we will get rid of the brackets based on the above rules. Thsi will give us:
12x³ + 7x + 3 - 3x⁷ - 4x

Now, we will combine like terms as follows:
12x³ + (7-4) x + 3 - 3x⁷

This will give us the final expression:
-3x⁷ + 12x³ + 3x + 3

Hope this helps :)