\(a,-7x^2\left(3x-4y\right)\)
\(=-21x^3+28x^2y\)
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\(b,\left(x-3\right)\left(5x-4\right)-\left(x+1\right)\left(2x-3\right)\)
\(=5x^2-4x-15x+12-\left(2x^2-3x+2x-3\right)\)
\(=5x^2-4x-15x+12-2x^2+3x-2x+3\)
\(=3x^2-18x+15\)
\(=3x^2-15x-3x+15\)
\(=3x\left(x-5\right)-3\left(x-5\right)\)
\(=\left(x-5\right)\left(3x-3\right)\)
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\(c,\left(2x-1\right)^2-\left(5x+2\right)^2\)
\(=\left[2x-1-\left(5x+2\right)\right]\left(2x-1+5x+2\right)\)
\(=\left(2x-1-5x-2\right)\left(2x-1+5x+2\right)\)
\(=\left(-3x-3\right)\left(7x+1\right)\)
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\(d,\left(x+3\right)\left(x-3\right)-\left(x+1\right)^3\)
\(=x^2-9-\left(x^3+3x^2+3x+1\right)\)
\(=x^2-9-x^3-3x^2-3x-1\)
\(=-x^3-2x^2-3x-10\)
