As we are differentiating a product (two things times together) we can use the product rule which is if:
y = u(x)v(x)
then
dy/dx = u(dv/dx) + v(du/dx).
So firstly looking at our equation we need to identify u(x) and v(x). In our case
u(x) = x3 and v(x) = sinx
Now we need to differentiate both of them seperatly so (remember when we differentiate we times by the old power and then subtract a power)
du/dx = 3x2 and dv/dx = cosx
Now putting all this into the formula we have
dy/dx = u(dv/dx) + v(du/dx)
= x3cosx + sinx(3x2)
Then rearranging this we get
dy/dx = x3cosx + 3x2sinx