+18 \(9c^2-(-3c^2-9c) \\\ \\ \text{Let's start by disbributing the negative:}\\\ \\ 9c^2+3c^2 +9c \\\ \\ \text{Now, you simply combine like terms.}\\ \text{This means, if the variable(}c\text{) has the same} \\ \text{power, then you can add those terms}\\ \text{together:}\\ \ \\ \underline{12c^2+9c} \)
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