expand by distributing the 6 and the - 4 and we get

18x^2 + 36 - 16x^2 - 20

combine like terms (ie the x^2's the x's and the constants) and we get

(18x^2 - 16x^2) + (36 - 20)

2x^2 + 16

or

2(x^2 + 8)

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