SOLUTION: Combine like terms. -9b - 4a + 3c + 3b - 9a

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Combining like terms means to add the ones that have the same variable(s) and the same power(s). In this case it would be a b and c.
for a: -4a + (-9a) = -13a
for b: -9b + 3b = -6b
for c there is only one: +3c
Final equation would be:
-13a - 6b + 3c