Answer:
How did they get the 1st step in the 1st place?
Answer:
How did they get the 1st step in the 1st place?
For the sake of having an answer...
They used the chain rule,
$\frac{dy}{dx} = \frac{dy}{d\theta} \frac{d\theta}{dx} = \frac{dy}{d\theta} \Big/ \frac{dx}{d\theta} = \frac{3\sin^2\theta \cos\theta}{-3\cos^2\theta\sin\theta}$