differentiate $\displaystyle 3x^5(2x-7)^4$ with respect to $\displaystyle x$.
Use a combination of the product rule and chain rule.
$\displaystyle \frac{d}{dx}[3x^5(2x - 7)^4] = 3x^5\frac{d}{dx}[(2x-7)^4] + (2x-7)^4\frac{d}{dx}(3x^5)$.
Go from here. You will need to use the Chain Rule to find $\displaystyle \frac{d}{dx}[(2x-7)^4]$.