y= 7x^2 cos3x (with respect to x)
i got as far as -sin3x and 14x but got stuck here!
Okay you can use that form. (It's the same thing anyway, and you should be able to recognise and use all the ways of writing it)
I'm taking your word for it that the derivative of cos 3x is -3sin 3x (Although it doesn't seem quite correct...)
Then:
$\displaystyle \frac{dy}{dx} = u \frac{dv}{dx} + v \frac{du}{dx} $
$\displaystyle = (cos \ 3x) (14x) + (-3 sin \ 3x) (7x^2)$