The process is similar to (and easier* than) adding three 4-digit numbers. Add the numbers in each column.
The sum is ...
(2+1-3)x³ +(-4+6+2)x² +(6-8-4)x +(-3+12-7)
= 0x³ +4x² -6x +2
The sum is 4x² -6x +2
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* The process is easier because there are no "carry" operations from one column to another as there may be when adding multi-digit numbers.