Factored:
3x2 - 21x = 3x(x - 7)
6x + 8 = 2(3x + 4)
x3 - 14x2 + 49 = <can't be factored>
15x + 20 = 5(x + 4)
To divide fractions, multiply by the reciprocal, so:
= [3x(x - 7)] / [2(3x + 4)] times [5(x + 4] / [ x3 - 14x2 + 49 ]
= [15x(x - 7)(x +4)] / [2(3x + 4)(x3 - 14x2 + 49)]
... nothing cancels ...