Given y = t, we find (dy)/(dt):

Given $y = \cos t$, we find $\frac{dy}{dt}$:

$$\frac{dy}{dt} = \frac{d}{dt}(\cos t) = -\sin t$$

Given $x = \sin 3t$, we find $\frac{dx}{dt}$ using the chain rule:

$$\frac{dx}{dt} = \frac{d}{dt}(\sin 3t) = \cos(3t) \cdot \frac{d}{dt}(3t) = 3\cos(3t)$$

$$\frac{dy}{dx} = \frac{dy/dt}{dx/dt} = \frac{-\sin t}{3\cos 3t}$$

The final answer is $\frac{dy}{dx} = -\frac{\sin t}{3\cos 3t}$.