16:
4b(5b-3)-2(b²-7b-4)
= 20b²-12b-2b²+14b+8
=18b²+2b+8
18:
3m(3m+6)-3(m²+4m+1)
= 9m²+18m-3m²-12m-3
=6m²+6m-3
20:
4x(3x+2)-3x×2x
=12x²+8x-6x²
=6x²+8x
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