6x^3 - 15x^2
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12x^2 - 60x
First factor out 3x^2 from the top and 12x from the bottom
3x^2(2x - 5)
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12x(x - 5)
Now you can divide 3x^2 by 12x to get x/4 (edit: Sorry I mixed up my numbers)
x(2x - 5)
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4(x - 5)
Now you are finally at the simplified form