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3x³-15x²-6x+30
Factor out 3, using brackets:
3[x³-5x²-2x+10]
Factor the first two terms in the brackets by factoring out x²
Factor the last two terms by factoring out -2
3[x²(x-5)-2(x-5)]
Factor out the (x-5) inside the brackets, leaving the x² and the
-2 to put inside a parentheses:
3[(x-5)(x²-2)]
Erase the brackets:
3(x-5)(x²-2)
Edwin