Use FOIL. (First, Outside, Inside, Last). Remember to combine like terms afterwards.
(6x - 9)(5x + 4)
(6x)(5x) = 30x²
(6x)(4) = 24x
(-9)(5x) = -45x
(-9)(4) = -36
30x² + 24x - 45x - 36
combine like terms
30x² + (24x - 45x) - 36 => 30x² - 21x - 36 (answer)
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(3x² + 2x)(x - 6)
3x²(x) = 3x³
3x(-6) = -18x
2x(x) = 2x²
2x(-6) = -16x
Combine like terms
3x³ + 2x² - 18x - 16x
3x³ + 2x² - 34x is your answer
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(2x² - 5x + 3)(4x³ - 1)
FOIL: Distribute each number in one parenthesis to all monomials in the other
4x³(2x²) = 8x^5
4x³(-5x) = -20x^4
4x³(3) = 12x³
-1(2x²) = -2x²
-1(-5x) = 5x
-1(3) = -3
8x^5 - 20x^4 +12x³ - 2x² + 5x -3 is your answer
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hope this helps