y = 3 cos 4x

Applying product rule:

\(\frac{dy}{dx}\) = u\(\frac{dv}{dx}\) + v\(\frac{du}{dx}\)

Where u = 3 and v = cos 4x

\(\frac{du}{dx}\) of 3 = 0 and 

\(\frac{dv}{dx}\) of cos 4x = -4sin 4x

\(\frac{dy}{dx}\) = 3 \(\times\) -4 sin 4x  + cos 4x \(\times\) 0

\(\frac{dy}{dx}\) = -12 sin 4x