r=cos[theta/3], theta=pi

So dy/dx is dy/d theta / dx/d theta and x=rcos theta, y=rsin theta and i found the derivatives of each which gave me:

x=1/3(-sin 2th/3)-2sin(4th/3)

y=1/3((cos(2th/3)+2cos(4th/3))

I'm kinda stumped here. I have dy/dx as a big mess but I don't know what to do from here.