By the chain rule,
[tex]\dfrac{\mathrm dy}{\mathrm dx}=\dfrac{\frac{\mathrm dy}{\mathrm dt}}{\frac{\mathrm dx}{\mathrm dt}}=\dfrac{4\sec t\tan t}{3\sec^2t}=\dfrac43\sin t[/tex]
When [tex]t=\pi[/tex], we have
[tex]\dfrac{\mathrm dy}{\mathrm dx}\bigg|_{t=\pi}=\dfrac43\sin\pi=0[/tex]
