\(=\left(4x^2-7x-50\right)^2-x^2\left(16x^2+56x+49\right)\)
\(=\left(4x^2-7x-50\right)^2-x^2\left(4x+7\right)^2\)
\(=\left(4x^2-7x-50-4x^2-7x\right)\left(4x^2-7x-50+4x^2+7x\right)\)
\(=\left(-14x-50\right)\left(8x^2-50\right)\)
\(=-4\left(7x+25\right)\left(2x-5\right)\left(2x+5\right)\)