a) $\left(3x-5\right)\left(9x^2+15x+25\right)$

$=\left(3x\right)^3-5^3$

$=27x^3-125$

b) $\left(2x+7\right)\left(x^2-14x+49\right)-2x\left(2x-1\right)\left(2x+1\right)$

$=2x^3-28x^2+98x+7x^2-98x+343-2x\left(4x^2-1\right)$

$=2x^3-28x^2+7x^2+343-8x^3+2x$

$=-6x^3-21x^2+343+2x$

c) $\left(4x-7\right)\left(16x^2+28x+49\right)\left(3x+1\right)\left(9x^2-3x+1\right)-9x\left(3x^2-1\right)$

$=\left(64x^3-343\right)\left(3x+1\right)\left(9x^2-3x+1\right)-27x^3+9x$

$=\left(6x^3-343\right)\left(27x^3+1\right)-27x^3+9x$

$=1728x^6+64x^3-9261x^3-343-27x^3+9x$

$=1728x^6-9224x^3-343+9x$